What happened in Twilight Breaking Dawn when Jacob imprinted on Renesmee?

A number of things:

1. The wolf pack was forced to make peace with the Cullens, because it's the wolf's absolute law not to harm anyone a wolf imprints on.

2. Renesmee brought the wolves and vampires together, and they became allies against the Volturi.

3. Bella and Edward were very upset, but eventually understood what happened.

4. Jacob became anything Renesmee needed - brother, friend, or guardian.