Complex sentence: NT stands for "Non-Tracking", meaning the head moves at a shallower slope to that of the tracks on the tape, crossing several during each pass, albeit only reading partial data from each one.
Split into simpler sentences: NT stands for "Non-Tracking", meaning the head does not precisely follow the tracks on the tape. Instead, the head moves at a shallower slope to that of the tracks on the tape, crossing several during each pass, albeit only reading partial data from each one.