It means after 100 hours, having no profit to show for it—100 hours of losing money. Example: Your expected value (EV) after 100 hours of play is $10,000. But after 100 hours of actual play, your actual value (AV) is -$1000. You have been on a 100-hour downswing.
Blackjack is a game of great variance, with 100-hour swings in either direction, up or down, which is very normal. It's only after hundreds of hours that you will see the "long run" edge of the game come into play and merge with your EV. But it's certainly not for the faint of heart, but very rewarding when compared to other investments and those who can seek out the hours. Of course, the games you choose to play will be the subject of what that all is.
Hope this helps. Cheers,