The important ones I can think of right now, especially for student skydivers are:
a thumbs up signal means to fall slower
a thumbs down signal means to fall faster
a hand signal representing a gun means to pull your rip cord right away.
if you hold your hand, palm up, with all but your middle and point fingers folded away (like a peace sign), and your middle and point fingers are against one-another and straight, means to straighten your legs.
if you hold your hand, palm up, with all but your middle and point fingers folded away (like a peace sign), and your middle and point fingers are against one-another and curved, means to bend your legs more.
if you hold your thumb and point finger together so that your thumb and point finger forms a circle, and your three remaining fingers almost showing the familiar three symbol, means everything is fine.
If you hold your hand palms up, and beckon a person closer with one hand, means to come closer
If you hold your hand, palm down, and beckon a person further away, means to move away.
If you hold a peace sign, palm up, but bring your middle finger and point finger closer to one another, it means to close your legs.