From the Super Mario Wiki
A Flower Tab increases Mario's maximum FP (Flower Points) by one in the game Super Mario RPG: Legend of the Seven Stars. Flower Points are needed to perform special attacks. Additionally, Flower Tabs recover all of Mario's FP.
Note: Flower Tabs need to be used on the item screen to increase Mario's maximum FP, unlike Flowers, which automatically increase Mario's maximum FP as soon as he picks them up.