EV in EV line means "estimated value". It is a mathematical calculation of how much money will you earn/lose in the long run going all-in the way you do.

Let's say you go all-in preflop for 5$ with A:sA

vs K:sK

and you lose it. You had 82,637% chance to win this hand and therefore you EV winnings are 0,82637*10 (10 is for $10 as you and your opponent put $5 in pot) which equals 8,27$. And you have lost 5$. Because of that your EV line is 13,27$ above your winnings line now. Basically if your EV line keeps going up on your graph, you are playing good and should continue, you will be winning in the long run.

Offcourse, that line never shows failed bluffs and much more situations which you can't really calculate that simply, so its results should be taken with reservation.

BB means big bet and it originaly comes from limit poker. It equals to 2*bb (big blind). If you are playing on NL5 BB=0,10$.

If your EV line is 800BB above your winnings line it means you should have won 1600 big blinds more than you actually did (mathematically).