In boxing, the "one-two combo" (also called "the old one-two") is a nickname of a jab and cross combo. It is so named because, in boxing parlance, the jab is often referred to as a "1" and the cross as a "2".
Boxers usually learn this combination before any other.
The jab is designed to get the fighter in range while effectively blinding the opponent and hiding the cross, which is a longer punch that will be slipped if thrown on its own. Generally, the jab is thrown with 60% power while stepping forward into range, and the cross follows at full power as the lead hand is retracted back to the guard.
Many boxers use this combination of punches to weaken their opponents during the beginning of a fight before any other combinations.