Let me break it down for ya.

First off, 100bb is the equivalent to 1 buyin. The reasoning is that is the maximum amount you can buyin on the table with (barring some stakes on certain sites). This is the people refer to stakes as say 25nl, or 50nl. Because on a .10/.25 table the max buyin is $25. And on a .25/.50 table the max buyin is $50.

The general rule of good bankroll management is 20+ buyins for the stakes you want to play. That is you need 20 100bb buyins. So if you want to play 25nl you would need 25 * 20 = $500. And imho 20 buyins is a little aggressive over 25nl, meaning it would be best to have more.

So at this time you are only rolled for 25nl (.10/.25 stakes). You wouldn't be rolled for 50nl (.25/.50 stakes) until your roll is $1000, but that would be cutting it close and it would be better to wait for it to be higher.

You wouldn't be playing $1/$2 (200nl) until around 5k-6k BR imo. And then only if you good enough to have beaten 100nl over a good sample size.