Stuffing is the art of interrupting an opponent’s attack by taking advantage of invincible hitboxes, quick startup, and/or priority. The idea is to beat them to the punch, rather than defending or trading hits.
This can mean stopping an opponent’s string.
The cases in which there is a pattern you can use after seeing the opponent’s attack are powerful indeed, but more often than not, you will have to have some air of prediction to it.
Patterns that use invincibility have a high chance of success, but many invincible moves leave you very open, so there’s something of a high risk.