Complex sentence: They are forced to stop after learning Jacob has imprinted (an involuntary response in which a shape-shifter finds his soul mate) on Renesmee, which, under their wolf law, means Renesmee cannot be harmed.
Split into simpler sentences: They are forced to stop after learning Jacob has imprinted on Renesmee. Imprinting is the involuntary response when a werewolf finds his soul mate thus under their wolf law means Renesmee cannot be harmed.