These Mushrooms can be used for immediate consumption (for healing purposes), for crafting (mainly potions), for cooking, and as a decoration on display.
Since update R59 on January 31st 2019, one Brown Mushroom can be processed into 1 unit of Brown Mushroom Spores that can be planted. 6-10 batches of Brown Mushrooms can be harvested then.
How to obtain
Brown Mushrooms, much like Glowing Mushrooms, can be found in the shape of cubic blocks with Mushrooms embedded into another material.
As for Brown Mushrooms, they look like blocks of Dirt with polypore mushrooms (much like turkey tails) inside here and there directly under most surface layers just above the Fossil layer among Dirt blocks.
To collect Brown Mushrooms, no Power Cells are needed. When pulling such a block with Brown Mushrooms inside, you will receive one (portion of) Brown Mushroom, but you will not be provided with any additional Dirt-blocks.
Moreover, Brown Mushrooms can be a content of Wood Treasure Chests that can randomly spawn in the night in dark areas on surface blocks, in Obsidian Treasure Chests that andomly spawn in dark areas on Stalactite layer blocks, in Iron Treasure Chests that randomly spawn in dark areas on Lava layer blocks, and in Diamond Treasure Chests that randomly spawn in dark areas on Corrupted Blocks.
How to use
Brown Mushrooms cannot be placed into the game world from your quickbar, instead they will be eaten by the player-character if used with right click (as the default keysetting).
However, you can indirectly place them by growing them from Spores (only in suitable biomes) and removing the soil under them.
Brown Mushrooms can also be consumed by dragging them with the mouse-cursor (while holding down the left mouse button) from the inventory over the character's 3D model to the right.
Brown Mushrooms - like other mushrooms - will heal 250-300 points (usually 272 of 1000 maximal health points on your the red bar at the bottom of the screen) after ca. 15 seconds (shown with a heart-symbol on the left side of the screen). Each second, ca. 17 health points are regenerated.
Cooldown: once a Brown Mushroom was consumed and the healing-effect starts, you cannot immediately consume another Brown Mushroom, but will have to wait for 30 seconds, more than necessary for the former one to finish its healing effect (15 seconds as mentioned).
However you can consume other mushrooms (Red Mushrooms, Glowing Mushrooms), Potions and/or Food while the healing-effect of the brown mushroom is still ongoing. You can also use the healing effects of Mineral Water and/or Healing Beacons while being healed by a Brown Mushroom at the same time.
A number of crafting-recipes demand "Mushrooms" of any kind, which means you can click on the arrows next to the mushroom icon in such a recipe in order to select either Brown Mushrooms or Red Mushrooms or the rare Glowing Mushrooms to craft the according recipe.
Brown Mushrooms are a type of Food that can also be put on display by placing them into the slots of display containers like Placemats, Stone Wall Shelves, Hidden Temple Altars, Galactic Grav-Chambers, Frozen Containers, Holiday Decorative Trees, Snow Buried Containers, Wood Planters, Flower Pots, Holiday Decorative Trees, Holiday Mantles or the like.
Brown Mushrooms are also a cooking ingredient that can be used in a Cooking Station to cook a large range of Food that will provide different buffs for player characters when consumed, but can also be fed to Pets.
Mushrooms of all types can be used for cooking Food like common Soup, Tar Soup, Bog Broth, Wholesome Soup, Blazing Soup, Corrupted Soup, Mushroom Sandwiches, Wholesome Sandwiches, Tar Sandwiches, Bog Sandwiches, Blazing Sandwiches, Corrupted Sandwiches, common Pie, Mushroom Pot Pie, Wholesome Pie, Blazing Pie and Corrupted Pie.
Planting Mushroom Spores
Simply carry Brown Mushrooms with you in your inventory and activate a Processor that has been placed into the world with right-click or "f" (as the default key) while pointing your mouse cursor at the Processor.
If these Spores are planted in a suitable environment with a unit of Water or other liquid adjacent to the block they are placed on, they will grow into 1 batch of Brown Mushrooms each after 10 minutes real time. Otherwise, the Spores will stay fallow.
Very cold biomes (like Taigas or snow-covered Tundras), very high altitudes (like the top of snow-covered Mountains over 150 blocks) and very hot biomes (like the Lava layer) are unsuitable to grow Mushrooms.
If fertilized with Pigsy Droppings, Brown Mushrooms will immediately grow from the Spores. Fallow Mushroom Spores cannot be fertilized.
Upon harvesting (which does not require any Power Cell to be equipped) 6-10 Brown Mushrooms can be obtained from the spot where 1 batch of Brown Mushroom Spores was planted, no matter what type of soil or if fertilizer was used.
Brown Mushroom Spores can be captured with a Capture Block. Such, you can buy more of them as part of building kits for Blueprints, and quite cheap as well - either Blueprints that you have captured yourself, or Blueprints that were made by other players and that you have subscribed to in the Steam Creativerse workshop, or Blueprints that you have customized with Brown Mushroom Spores.
Building kits can be paid with ingame Coins that can be bought with real money via Steam Wallet in the Store. To buy a building kit, either select the according Blueprint or activate a cornerstone that has been placed into the game world (and has optionally been customized). Check the block kits first by clicking on the description "Built with xxx Blocks" before you'll click on the green price tag button below "Get kit and build now".