The term Mean Free Path is used in the physics of moving Particles (atoms, electrons, Photons, etc.) to indicate the average length they travel between interactions. When performing calculations in a substance where some type of particle moves around, estimates of the consequences can be calculated by assuming the particle always moves the same distance between interactions, using the mean free path as that distance. The mean free path can be determined from density, and the radius at which the particles interact.
Some situations where it is used:
Optical Depth (τ)