What's a wave dash? A wave dash ("wavu wavu" as many call it) is an advanced movement option that consists of consecutive crouch dashes being chained together and has unique moves that can be done from it. Wave dashing only available to certain characters. Specifically, Kazuya, Heihachi, Devil Jin, Jin, Bob, Hwoarang, Armor King, and King can wave dash (side note: King's wave dash has a unique input from the rest and is generally considered to be harder to do).
How do I do a wave dash? To perform this technique, first make sure you have a consistent crouch dash motion (f,n,d,d/f) as this will be the basis of the wave dash. To do the actual wave dash do the crouch dash but end with a f after the d/f (a helpful tip is to think of it as doing f,n,qcf), then simply rinse and repeat, making sure to return to neutral in between. So your command history should look like f,n,qcf,n,f,n,qcf,n,f,n,qcf... so on and so forth. Sounds simple enough, but getting a consistent, fast wave dash is something that takes substantial practice for most players, and don't forget you should be able to do it on both player 1 and player 2 sides.
How is this even possible?
The reason wave dashing works is because hitting forward during the crouch dash cancels into a regular dash (you're buffering the first forward from doing the qcf so you only have to press it one more time). Then you've already hit forward so now you just have to do qcf to get the next crouch dash.