Rebeat is surprisingly simple. If you do a combo with a UNIST character, you can press one of the lower normals to Rebeat. I guess a better way would to show a combo route that only a UNIST character would be able to do:
So for example, you're playing Hyde and you do 2C, you can then rebeat that into any of his other buttons, 5A, 5B, 2B, etc. That's what Rebeating is.
So one could be involving Gordeau (Sorry for using him so much, I used to main him for 18 months, rip me). He could do 5BB>2C>5AAA which is a rebeat. This has also been in Melty Blood as well, one of French Bread's other fighters.
And one example involving Linne can be 2A>2B>2C>5B. Rebeating can also make unsafe options safe on block.
And of course, Gordeau will be the example again. His 5B is extremely unsafe on block, which is where 2A comes into play. 2A used to make 5B very safe, however due to his nerfs, it isn't as safe as it used to be. So not only can you use Rebeating to apply pressure, but you can also use it to make yourself safe on block. Though I should say that most moves the UNIST side have are mostly unsafe on block, however some of them allow you to jump cancel on block, so always use that opportunity to get out of there to reset neutral, or hit them with a high or mixup.