It means that something happens the same amount of time that it doesn't happen. For example, you have a friend who is late half of the time. Half the time she is on time. And half the time she is late. She is late as often as not she is not late or "She is late as often as not." You can also say, "She is often late." but I don't know how often she is late. She could be late half the time, but she could also be late 25 or 30 or 60% of the time.

There is also "more often than not" which indicates somewhat higher frequency than "as often as not". It's good to remember them together :-)

Thanks!~ Gary! Although I know nothing about baseball, but I can understand what you say, and it make have a better understanding of this phrase, thank you!

Good question. I think of this as "it is as frequent as not being frequent". This is literally what it means, and it refers usually refers to a noun or pronoun. For example, if I'm watching a baseball game and the pitcher is throwing as many strikes as he is balls, then I might say...."He is throwing strikes as often as not." Therefore, the "not" is short for " He is throwing strikes as often (as he is throwing balls.)