This is an emergency first aid kit you can use to quickly heal yourself by a small amount during combat.
Left-click the first aid Kit to pick it up, and place it in your shortcut bar for easy access.
To use the first aid kit on yourself you must press F1 to target yourself.
Right-click, or press the appropriate key (1-10 depending on where in the shortcut bar you placed the icon), in the shortcut bar to use the kit.