Basically, Zone 1 is for Premium seats and Platinum/Gold members.
Next is exit row and young families.
Zone 2, back rows.
Zone 3, middle rows.
Zone 4, front rows.
Zone 5, you’re sitting in the back bathroom.

Of course, you’re boarding process will vary by airport. At my home airport of YYJ they will often just call Zone 1, exit rows, and Zone 2 at the same time which leads to a recreation of the Battle of Helms Deep for overhead luggage.