What does it mean when a horse keeps tripping when riding?

You need to find out why he is tripping. Does he trip when he is not being ridden? If he is then it could be a physical problem like Arthritis. He may not be able to pick his feet up properly because he is in pain

Another possible cause is that his feet are too long, have a farrier look at them if this is the case.

How old is the horse and how hard is he being ridden, he may just be tired. If you are really concerned have him assessed by a vet while he is ridden.