Why do we need the "mean" in mean free path?
In every context we talk about "mean free path", we talk about particles. Particles usually bounce around a lot, from collision to collision. In between two bounces a particle moves in a straight line. That is the expected behaviour from Newton's laws of motion.

Between one collision and the next, a particle therefore moves a distance. That is the "free path". So why do we need to include "mean"? The thing is, you can never be sure about how far the particle moves after a collision. It might as well immediately bounce into another particle, as well as move a considerable distance. However, if you measure really many particles, their average is going to be pretty stable and predictable. That gives a nice, usable number. The "mean" here is for average. (In case you wonder, yes, this applies to optics too, but the concept requires a little more abstraction).