To bash is to hit something hard. You might stand up suddenly on a sailboat and bash your head on the mast.
Marching band drummers bash their drums with mallets, and terrible drivers bash their bumpers into trees. You can also figuratively bash another person by saying terribly critical things about her. When bash is a noun, it means either "a hard hit or blow," or "a loud, festive party:" "Welcome to my bash! Don't bash your head on the piñata."