Cam lean is when one or both cams is not in line with the other cam/idler wheel and/or string. There are two main reasons for cam lean.
1. Worn out bearings or bushings
2. One side or your split yoke on the buss cable is too long or too short.
Cam lean can cause a bunch of different problems, but mainly...
1. An awkward position for your center shot
2. Accessive serving wear at your cams
3. Inaccuracies because of sloppy play in your bushings if they are worn out.
Check your cam lean by placing an arrow flush against both cams/idler wheel and run it down along your string. When the arrow is flush against the cam/idler wheel, the arrow should extend right down, along side of the string...parallel with the string. If your arrow runs into the string or away from your string as you get closer to your nock point, check your split yoke and/or your bushings.
Hope this helps!