Allowing the dough to prove just means to rest it and give it the opportunity to rise. Over proofing can produce unstructured or uneven bread. So the 1st prove should only be for around 1 hour. Knocking back (hitting the risen dough with your hand) removes the air meaning the dough can be handled again and shaped. A second prove allows the shaped dough to rise again, this usually requires less time (around 30 minutes).