What he is saying is that the rear derailleur may need adjustment or the hanger holding the derailleur to the frame may be bent. It could also be the derailleur or pulleys are not aligned properly. Lots of things happen under drive train stress, which makes diagnosis hard when we don't have our hands and eyes on the bike. I would suggest taking it to a shop. Willing to be proven wrong, but I doubt you will be able to fix this yourself.