An option select is basically a series of inputs that make the game choose the best option for the given situation. Crouch teching is probably the most basic option select. To perform this, you press LP+LK together while crouch blocking an opponent’s combo, and the game will choose the best option.
For example - Let’s say Balrog is in your face and he’s trying to hit you with cr.lp x 3. (Three crouching low punches) The crouch tech option select defeats some of his options.
He could try walk up and throw you after one of those punches. But if he takes to long before he throws you, one of your cr.lk’s will come out and hit him. Try it out in training mode - If you crouch and press LP+LK, a cr.LK will come out.
Same as #1, but if he catches you with the throw, you will escape the throw since you teching throws is done by pressing LP+LK.
If he doesn’t decide to go for a throw, you will continue blocking since you won’t be able to attack in between his combo.
Of course, this is just one example of a very basic option select. There are many option selects in the game that are used for different situations. A option select that is a bit more advanced is the option select sweep, as was demonstrated in the video that was linked. So lets say you keep knocking someone down, and you notice that they really like to backdash on their wakeup. The option select sweep is there to punish that.
Okay, lets use Akuma as an example. To perform this option select, you will have to time a cr.LP with you opponents wake-up, and after that perform a cr.HK and another cr.LP. Let’s say you knock down Ryu - let’s break it down how this option select works when performed correctly -
Ryu gets up and blocks - Your cr.LP will be blocked and your sweep will not come out since you will still be in recovery frames of your cr.LP. The second cr.LP will hit and you can continue your blockstring or go for a tick throw.
Ryu gets up and backdashes. If you perform the option select correctly, you will see that your first punch will whiff, and your sweep will come out, knocking Ryu back down. This is because you will recover from your jab in time for your sweep to come out and knock Ryu back down.
Ryu gets up and does nothing - This probably won’t happen often, but you will hit Ryu with a cr.LP. The sweep won’t come out, but the next cr.LP will, allowing you to go into a BnB combo.
Of course, not all option selects are bullet proof. Crouch tech can be beaten by a frame trap, and the option select sweep can be defeated by a move with invincibility.
It may be a bit confusing at first, but I hope this helps.