[M&M Guy]

Just to expand a little on JRut's answer -

The primary is the "Where should I be watching?" area.

The secondary is the "There's nothing in my area, so where can help out?" area. An example could be if you're the C, and all the players are over in T and L's areas. You could help by watching for illegal screens. Another example could be as T, you could watch for the secondary defender on the post player, while the L is watching the main matchup. The hard part is to not ball-watch, and still trust your partners to make the calls in their areas, but you're helping out because there's nothing specific to watch in your area.

These are just examples, and I'm sure there are more, but hopefully it gives you an idea on the principle.