As a concept, this involves anything that’s retaliating against an aggressor. A common example: blocking their poke attempts and immediately striking back (with shield grabs if they’re close, with neutral attacks or your farthest-reaching pokes if they’re not). Another frequent scenario: you’re sparring with someone, exchanging feints and pokes, and they whiff something really laggy like an up special or smash attack, giving you a clear opening to rush in and hit them before they can act again.
But some counterattacks are specific actions. Many fighters have some kind of special parry or countermove, which usually absorbs or deflects an incoming strike before lashing back. Fighters like Incineroar, Peach and Daisy, the Fire Emblem warriors, Shulk, and Bayonetta all sport counterattack specials, among others.