It's just a term. The point is he hit a wall in the second half of the season if you would like to be more literal.
In the first half of the season he had 26 receptions for 444 yards and 4 touchdowns.
In the second half of the season he has 18 receptions for 292 yards and 1 td.
So far through 4 games however he is on pace for a half season that looks like
28 receptions, 358 yards, 8 tds
the touchdowns look impressive, but we still need a lot more from him in terms of yards and receptions. Hopefully with Romo getting healthier we see that from him.
Williams should get about 100-120 targets for the season. Right now he is only on pace for 80... It's not enough.