What does it mean if two horses are nibble each other?

Yes, it does mean they get along well if they nibble at eachother. It is a form of grooming they use, so it feels good to them.

sometimes yes or they may just be grooming each other or if one is running they may be just putting each other in place

Yes it is because it is just normal behaviour- it is mutual grooming, and it builds a bond.

They're not nibbling: they're grooming each other. Grooming is one of the social bonds that horses use to become and stay friends. Horses are social animals.

Horses will groom each other if they get along. This action looks alot like nibbling, but they don't do it to be mean. However, a horse who goes to nibble at the behind of another passing horse...