The scrotum holds the testicles. This is a very sensitive and a very delicate part of any man's body but remains a highly erotic area. Applying too much pressure to the testicles will cause pain. Just keep in mind to go very gently.
Play with the testicles gently with your hands. You can also suck or lick the scrotum and if you put them into your mouth (with the minimum of pressure) you will see your partner lost in the moment.