Refreshroom is a type of Mushroom that refreshes your lives during the events of Mario & Luigi: Partners in Time and Mario & Luigi: Bowser's Inside Story. In the former game, the item is a one of a kind and cures the state of "shroomshock", a state of unconsciousness. Here, it would be in Toadsworth's room in Princess Peach's Castle.
However in Mario & Luigi: Bowser's Inside Story, they only appear in the fight where Bowser significantly enlarges to fight the big bosses; they drop them at a larger size than usual to match Bowser's size. Refreshrooms heal 50% of Bowser's health bar in these fights.
The word "refreshroom" is a portmanteau of "refresh" and "shroom". However, some people may read the word as "Refresh room".