The perineum is the skin between the vagina and the anus. Normally, this is definitely a part of your anatomy that you would hardly pay any attention to at all. But during childbirth, it will become one of the tissues that is most stretched. When your baby is passing through the birth canal and about to emerge, this is the skin that is at risk of either tearing or being cut by one of your birth attendants (this is called an episiotomy). Some midwives, doctors, childbirth educators and mothers say that gentling massaging the perineum in the last few weeks of pregnancy can help it stay intact during birth. Not tearing or being cut will definitely help you recover from childbirth a bit more easily. Tears are no big deal I can say from experience but peeing can hurt in the first week after you give birth. There is no doubt that it is more pleasant to avoid, if that is possible.
Perineal counter pressure during birth is something that can be done in addition, or instead, of perineal massage during the last weeks of pregnancy. If you choose this option, someone will put pressure on your perineum as your baby is crowning. This will most likely be the person who will be delivering your baby, either an OB or a midwife. But birth doulas, or your partner, can also sometimes provide counter pressure. Does this really prevent tears? There is no clear evidence about that, so you will have to decide for yourself. Some women don't like the sensation and find it distracting during the crucial phase of pushing their baby out. Others swear it helped them, and think it provided some pain relief as well. Other things you might like to read about are drinking raspberry leaf tea in pregnancy and Kegel exercises during pregnancy.