If a party opponent — say, the plaintiff in a case where you are the defendant — makes an admission or confession statement, you can introduce that statement into evidence.
For example, suppose that you are involved in a motor vehicle accident. As you’re speaking to the plaintiff after the accident, the plaintiff apologizes to you and admits that they were speeding and were distracted by their cell phone at the time of the accident. This evidence would not only be admissible, but would also be very helpful in undermining the plaintiff’s injury claims against you.