Stones are low if they are on or below the third line.
It is a mistake to have too many low stones: they are good for building eye space, but the second line isn't good for taking serious territory. Plays on the fourth line have much more influence towards the centre. It is taught that one should aim for balance.
Of course it is not too late for Black to improve his potential and for White to destroy hers. If Black had gone for a few second line plays he would really be in trouble however.