I'd just like to add, and this is a good convo going on, that a route tree would look like a stick figured tree if you drew it on a piece of paper. Some offenses have each route numbered, like a #9 is a corner route in this example:
Even numbered routes usually are
INS, and
odd numbered routes are
OUTS.
Here's another example of something closer to what we run:
Now, usually when they say the guy is a bad route runner, what they mean is that he is inconsistent in WHEN he gets from point A to point B, because the QB has to go through a progression after the primary read, or call is taken away. A route should be run the same exact way every time. Same steps, cut off the same foot, etc, so that the QB knows where you will be in the route if he has to throw it your way.
So the QB will look to the next guy in the tree, and now he is starting to run out of time. A lot of these routes are timing routes and the QB throws to a spot where the receiver
will be, so if the receiver is not there, because of a bad route, then the QB will miss him, and most people put the blame on the QB, but that's not usually the case.
When they say that a receiver can't run the whole tree, they probably mean that he doesn't have the speed to run certain routes. Some receivers can't get off the line of scrimmage, and this will screw the timing or he might be pushed or run himself out of position.
Most teams run option routes now, which was mentioned earlier, to where the QB and the receiver have to make the same read on the coverage, and the actual route will be determined by that.
If you want to know anything more, please feel free to ask. There is a lot of knowledge on this board.