Gas molecules are always moving very fast and traveling in completely random directions. They constantly crash into the solid walls of their container and into each other. The mean free path measures the typical straight line distance between these frequent physical crashes.
This specific distance depends heavily on the internal pressure and temperature of the gas system. If you pump gas molecules out of a container, the internal pressure drops very quickly. This leaves fewer molecules in the space, making the gas much less crowded overall.
The remaining particles can now travel much longer distances before they hit a random neighbor. The physical size of the actual molecules also changes this average flying distance. Large, bulky molecules will bump into each other much more often than tiny atoms will.
Students often forget that temperature also plays a major role in this exact physical rule. Heating a gas at a constant pressure makes the gas expand into a much larger volume. This volume expansion spreads the molecules further apart and increases their safe free flying distance.
Understanding this basic concept helps chemists design complicated vacuum chambers for their lab experiments. It also helps working scientists predict exactly how fast two different gases will mix together.