300 BB?

Im new to limit hold'em but I have played NL for a long long time

One of the most important things about this game is making sure you are playing at the right table at the right stakes so that you are not playing w/ 'scare dmoney' I have heard that 300BB is appropriate for limit holdem but does that mean

Big Blind
or
Big bet?

I played 1/2 blinds with 300 bank roll and turned it to 600 in nearly 2 days. I know that I have been very lucky, but was i playing over my head?

So pretty much just define big blind

thx