What does this advantage statement signify? In many Attribute Based Encryption papers I see this line...

The advantage of an adversary A in this game is defined as Pr[b' = b] − 1/2.

Can someone explain what it means to me?

I assume it means that the chances of the guess being correct is equal to 1/2. Is this assumption accurate?
It means exactly what is written, the advantage of the adversary is a quantity which is defined as the difference between the probability that $b = b'$ and $1/2$. For example if the probability that $b = b'$ is $3/4$, then the advantage of the adversary is $3/4-1/2 = 1/4$.