Every time you hit the opponent, you put them into a certain number of frames of hitstun. If you hit them again while they're still in hitstun, there's (typically) nothing they can do about it, and that's called a combo. Hitstun decay is a mechanic to prevent infinite combos by making it so that, as the combo goes on longer, each hit does less hitstun than it normally would until the combo eventually drops, and the character getting hit can "tech" out of the dropped combo with some invincibility. Here's a good explainer on how infinite combos are prevented and have been prevented in the past, as well as some of their drawbacks; the video author calls "hitstun decay", "hitstun scaling" instead.

I don't like hitstun decay for a few reasons. For one, it's not easily observable. If you've got a combo limit meter (like Skullgirls, Killer Instinct, and the upcoming Invincible Vs), you can see how much the move you just used has gotten you closer to the limits of the combo. It's not intuitive for a player to track how close they are to the combo dropping with hitstun decay. So because of this, you're basically just memorizing combos. If you land a hit with a move, or in a situation, that you haven't practiced, you have no idea how to guarantee that you can finish the combo, which means that if you're improvising, you're just quickly routing your combo into a knockdown. As a player, I hate memorization, and as a spectator, I hate watching a game that has just a few bread and butter combos and quick routes to knockdowns when they don't know what to do. I do like one game with hitstun decay, Guilty Gear XX Accent Core +R, which at least allows you to do "tech traps", where I'm expecting my opponent to air tech, and if they do, I get a new combo for free, so there's a mind game there that most games with hitstun decay, in my experience, don't have.

Hitstun decay is, by and large, the most prevalent form of infinite combo prevention in games with big combos, but it's the one I dislike most. Guilty Gear Strive, Mortal Kombat, and Tekken all use juggle decay, or gravity decay, where the opponent just falls harder and harder until eventually they hit the floor, and you can't combo them anymore. This is, of course, much easier for everyone to observe. My favorite method though is just using a meter to limit combos, because it allows for something much closer to freeform jazz. Every combo in Killer Instinct is different, because if you use the same combo every time, the opponent can break it. In Skullgirls, you're usually unable to do enough damage in a single combo with its limits, so instead you're looking to tactically drop your combo and sneak in a new one, which is called a reset.