From the Super Mario Wiki
The Mushroom Space is one of the spaces from the Mario Party series. It first appeared in Mario Party, in which it allows players that land on it to hit a roulette block showing a Mushroom and a Poison Mushroom. If the player gets Mushroom when hit, they roll again, but if it's a Poison Mushroom, they lose their next turn.
In Mario Party 4, it returns and allows players that land on it to get either a Mini Mushroom or Mega Mushroom. Each mushroom is hidden at random inside one of two boxes, one large and one small, that the player must choose between. If they already have three items, the space does nothing when landed upon.
In Mario Party 5, players could only create Mushroom Spaces by placing a Mushroom Capsule, Super Mushroom Orb, or Cursed Mushroom Capsule on a space. Anyone that lands on it gets whatever Mushroom is inside.