Easiest way to imagine it is this: the wind is not homogeneous. It oscillates (sometimes asynchronosly) in both direction and velocity. The more generic terms may include "puff", "pressure", "veer", etc.

If you stood at the top of a bridge or building and looked across an expanse of water, you can see some of the vagaries of the wind as it moves across the water. It's harder to visualize while you're on the water, but that's where the really good sailors/tacticians are able to shine.

A good lane, ideally, is one which gives you the best direction and pressure toward your intended direction, as well as no "spoil" from other boats to windward. The lane will most likely be in constant flux, and it's your job to try to stay "in phase" with these changes.

Or, you could be like me and chase flyers and get out of sync which ensures that I won't have to make space in my sailbag for any trophies.