It describes a delivery from the bowler that pitches short of the optimum length.
Length in cricket defines where the ball pitches on the wicket.
[1] A good length ball is one that pitches at a distance that makes it difficult for the batsman to ascertain whether to play the ball on the front foot or back foot.
This delivery can be dangerous for a batsman as it can bounce higher into the midriff.
Good exponents include Stephen Harmison and Glenn McGrath.