Suit establishment for the defence
South leads a club against 3NT. Each time the defence win a trick they play another club.
South led his longest suit. North returned the suit. Together, North and South defeated the contract. Nice.
Notice that the defenders must be patient and keep attacking the club suit at every opportunity. It's important for South not to take the aces in the other suits too soon. That would simply set up declarer's long suits.
Look again at the example on the previous page and you'll see how similar it is. Whether you are declarer or defending, the principle is the same; set up your long suits!
Returning partner's suit
You'll probably have heard of many bridge rules of thumb like "third hand high, second hand low". "lead your longest suit against 3NT". "always return your partner's suit". You shouldn't be following those rules just because you're supposed to or your teacher told you to. There should be a reason.
Some people think you should return partner's suit because it's ettiquette. Or partner's going to be upset if you don't. Almost as if it's some sort of bridge crime not to return partner's suit. Those are all bad reasons to return partner's suit. There are good reasons. Return your partner's suit if you think that's going to help your side win extra tricks.