Marvel vs Capcom 3/Character States/Floored State
From Shoryuken Wiki, Marvel vs. Capcom 3 Strategy and More!
Floored State occurs whenever you get knocked down to the ground by the opponent. There are technically three different categories of Floored States: Hard Knock-Downs, Soft Knock-Downs, and Forced Quick Rise.
In a Hard Knock-Down State, your character will get knocked to the floor and be stuck there for a fixed amount of time, which is different depending on the move that knocked you down. After that time, your character will automatically get up off the ground in place. However, if you hold Towards or Back on the controller before you get up, you'll roll forward or backwards respectively. The problem is that, while you are lying on the ground before you get up, the opponent can tag you with any OTG move that will pop you up off the ground that allows them to continue comboing you. Essentially, lying on the ground is an extended Hit Stun.
Some characters just have moves that will cause these Knock-Downs such as Wesker's Cobra Strike. But where you'll see it the most is from Air Combo Finishers. These cause Hard Knock-Downs that last a very significantly long time because you hit the ground and bounce first. During that entire bounce and through the entire time lying there, you can be hit by any OTG move. So if you can find a combination of Hard Knock-Downs and OTG moves, you can perform some very long combos.
For Soft Knock-Downs, your character will bypass the entire portion of lying on the ground and, the instant you touch the floor, your character will perform a mini roll backwards and be standing right away. That means you cannot be comboed by any OTG attacks. Also, if you hold Towards or Back before you hit the ground, you'll go into an immediate Roll.
Many Sweeps, such as Akuma's, cause Soft Knock-Downs. Also, anytime a hit tags an opponent out of the air very close to the ground and their air Hit Stun drops them to ground level, a Soft Knock-Down kicks in. And there are just some moves that cause Soft Knock-Downs, such as Ryu's ground Tatsumaki Senpukyaku.
Forced Quick Rise
In Forced Quick Rise, you'll hit the ground and immediately perform the short Backwards Roll, but you won't be able to do anything else. So it's just like the Soft Knock-Down, but you cannot roll forwards or backwards when you hit the ground. There aren't a lot of moves that cause this, but an example of this is Wesker's Light Attack version of the Mustang Kick. Once you hit the ground, you are forced into a small backwards roll and immediately stand up afterwards.