Prove that PP is closed under complement
$\frac{1}{2^{f(|x|)}}$ is the granularity of the probabilities. The probability distribution is over the possible outcomes of the coin flips. If you flip two coins for example, the probability that you accept can't be exactly 0.501. It's either 0 (you never accept), 1/4 (one coin flip combination leads to acceptance), 1/2 (two combinations make you accept), 3/4 or 1. 

So if you require the probability to be greater than 1/2 you know that it's at least $1/2 + \frac{1}{2^{f(|x|)}}$.