A funeral is a ceremony to bury or cremate the dead. If you go to a funeral, wear black and try not to seem like you're having fun.
Funeral comes from the Latin root fūnus, for "dead body," and so a funeral is the ritual that surrounds taking care of a dead body. Depending on the dead person's beliefs, there will often be a representative of a religion there to honor them in a traditional way. Sometimes we use funeral for dead things other than dead people. If you just gave a really bad party, you could call it a funeral for your social life.