What is message collison?

In a typical serial protocol, when mutiple nodes attempt to access the bus at the same time, a collison may occur, what is this collison exactly mean?

The multiple messages try to signal at the same time. Tell me the protocol, and I'll describe what happens at the voltage/current/radio wave level.

Irregardless of what happens on the bus itself it can most simply be understood as what happens when two people are talking at exactly the same time when their voices are heard with exactly the same precedence. Understanding of neither talker can occur.