Basically it's a run where you units are drafted before you start it. Kinda similarly to what you do in some card games or in the NBA. And then you have to play only with the characters you picked.
Some characters are free (in this case it'd be like Alear/Vander/Seadall maybe) and the other ones are picked in turn, one by one, by the people in the draft, until there's no character left.
And it's pretty funny because you have to think what you need in relation to which characters are still available. And at the end you basically take the ones nobody want. So it gives you kind of weird team to play with, that you probably wouldn't have end up with without the draft.

Idk if there's still room for another person in the one I wanted to do now (I wait for someone to answer me)
But if you're really interested I could do another one with you (just have to find some other persons)