If you have played Dota then you should know what the mode means.
But aint in the game yet.
About alt pick.
Its about balance to the picks.
Normally you see people waiting till the time runs out and then pick heroes.
When you do Alt Pick you are forced to pick. so means first someone on the legion or hellbourne chooses 1 hero. then it switches to the other team and then 1 or 2 of them need to choose a hero.
This way you can basicly choose counter heroes without it being unfair for the other team.