BB/100 is how many Big Blind`s you win on average per 100 hands, the best way to track this is to use a program like PT4 or hold`em manager that tell you your bb/100 winrate
Hope this helps

Is this for cashgame only? And only the position Bigblind?

Yeah, it's used mostly for cash games and it's not limited to the big blind position at the table. Win rate is basically a tool that poker players use to evaluate how skilled they are based on their past results. For example, if an NL10 cash game player managed to win 200$ in a span of 40,000 hands he won it at a rate of 5 big blinds per hundred hands he played (of 5bb/100 for short). Since big blind at NL10 is equal to 0.10$ it means that 5bb/100 player will on average win 0.50$ every 100 hands, 5$ every 1000 hands, 50$ every 10,000 hands etc. He can now use this information to predict his future results, inform his decision about bankroll management strategy etc. etc. Of course, it's only an approximation since in actuality the variance in poker makes it so you can never be exactly sure what your win rate is (and you need at least 100,000-200,000 hands to make an educated guess that's worth a damn), but we have to base our decision on something so this is what we use.

Thanks!