The route tree details basic wide receiver routes to run. It also incorporates a numbering system to each route.
The numbers can be used in playcalling to keep things simple.
For example, a coach could send in a play call for 3, 4, 4, 6.
This would begin with the WR closest to the sideline the coach is on. The closest WR would run an out (3), with the next closest WR running a curl (4), etc.