What does mutation mean in lefft for dead 2

Mutation doesn't mean anything in L4D2. It is simply an idea by the creators to mix things up a little. You need however, to get "The Passing (DLC) in order to access any of the mutations.
These mutations are just for the laughs, and feature some crazy game modes to really mix up your game play. For example, one mutation is where there is no health packs, adrenaline or pills, but there is a gnome, where you hold to gain health. However, while equipping this gnome, you are unable to use your weapons and can only knock back common infected along with a few weak uncommon infected. It can get crazy and survivors will often fight each other to get this gnome to heal themselves.