When liquids spill, they accidentally flow or dribble out of a container. You can also spill solid objects, like when you slip and spill the contents of your backpack all over the sidewalk.
When you spill your hot chocolate, the spot it makes where it lands is a spill. The act of spilling it is a spill too: "Sorry about the spill, Grandma." You can take a spill yourself, like when you tumble to the ground from your bike. Spill also means "talk," especially in the context of "spill the beans" or "spill your guts," both basically meaning "tell the whole truth."