"What is the significance of this quote to the plot of the play? "Do not give dilliance too much the rein; strongest oaths are straw to th' fire I' the' blood

Like a good father Prospero cautions Ferdinand to be good to his daughter. He tells him to be honorable to her. He tells him to be gentle with her and not get angry. Acting rashly will make him forget his vows. Prospero probably knew this might happen in his plan to bring them to the island. Really Prospero was never out to hurt Ferdinand. He wanted to make sure he was a man of honour.