Whenever you are asked to analyse the importance of a specific quote or group of lines, the place to start is by reading the context of those lines: what happens before and after. This is going to be something that will help you work out the significance of those lines in particular. If we look at the lines you have highlighted, we can see that Oberon utters them to Puck just after they have observed Hermia and Demetrius and Helena and the chaos that has befallen them because of Puck confusing the situation further. Now both of the male Athenian lovers love Helena and none love Hermia. Oberon is rightly angry with Puck for having disobeyed him and (intentionally?) mistaking the Athenian males. Note the full quote of what he says to Puck:
What hast thou done? Thou hast mistaken quite
And laid the love juice on some true love's sight.
Of thy misprison must perforce ensure
Some true love turned, and not a false turned true.
Oberon is basically telling off Puck in these lines, saying that as a result of Puck's "misprison" or mistake, a true love has been altered rather than a "false love" being made "true." For me, the importance of these lines lies in the question that every director must answer. Did Puck mistake the Anthenian lovers deliberately? Is he the kind of character that likes chaos and mischief so much that he has intentionally made the situation more complex so that he can enjoy it? How would Oberon respond? In what tone of voice does he utter these lines? Thus this quote is very important in terms of the relationship beween Oberon and Puck and reveals a lot about Puck's character.