To spit is to propel saliva—which is also called spit—from your mouth. Generally speaking, it's rude to spit, particularly if you're at a fancy dinner party.
There are some parts of the world where it's socially acceptable to spit in public, and it was once accepted in most of the West too. During the Middle Ages, Western Europeans found it less polite to "suck back" spit than to, well, spit. And spittoons, receptacles for spitting in, were common until the early 20th century, when the influenza epidemic and public health campaigns about tuberculosis greatly reduced public spitting.