This refers to the ever-so-slight vulnerability window the first time a player grabs a ledge during a given airborne period. Although the first ledge hang of a jump is intangible, the first two frames of hanging are vulnerable to punishment.
This tiny window is there to give edge-guarders a chance against savvy recovering fighters who zero in on a ledge handhold, seeking out the intangibility (and the restocking of their midair options after touching the ledge).
The best kinds of attacks to aim at the ledge right when recovering fighters surge toward it: projectiles dropped/fired around the ledge, melee swings with long active periods and hitboxes that stretch under the floor, and low-altitude downward-aimed aerial attacks. Two-frame punishing requires a lot of experience and is prediction-heavy, since it’s hard not only to react to an opponent going for a ledge hang, but also to recognize which move they’re using, altering their timing.
Strong two-frame punish-attack examples include lengthy active attacks like Bowser’s Fire Breath, PAC-MAN’s Fire Hydrant, Rosalina’s down air attack, and “z-dropped” items. Fighters without these types of attacks can use low-hitting smash attacks, such as Samus’s down smash and Mario’s down-angled side smash.
This mechanic is occasionally referred to with a verb. For example, “two-framing” someone means hitting their ledge-recovery window. See recovery and edge-guarding.