Relegation is like a promotion, but backwards. It's not something we have here in the US but is more familiar to soccer leagues in Europe where teams get knocked down to a lower league based on poor performance (i.e. if a AAA minor league baseball team that got last place were moved down to AA the next season).
It's a best of 3 series you play, and if you lose 2 out of 3, you are moved down to a lower division. If you win, you get to stay in your division. They work the same as other matches as far as training goes.