Meaning of “get behind” - Learner’s Dictionary

get behind

phrasal verb with get us uk /ɡet/ verb present participle getting, past tense got, past participle got, US gotten

If you get behind with work or payments, you have not done as much work or paid as much money as you should by a particular time.

(Definition of “get behind” from the Cambridge Learner’s Dictionary © Cambridge University Press)