Pick Me Up
From the Super Mario Wiki
When a party member's HP reaches zero, they fall to the ground, unable to battle. Thus, Mario or another one of his allies should use a Pick Me Up to revive the party member, restoring all of his or her lost HP. This item can only be used during battle since any fallen party members after a battle automatically regain one HP. Though Toadstool's Come Back move has the same effect, after learned, the group should still have a few with them in case she runs out of FP and a member falls, or the princess herself faints.