Yeah, Brokenstars is spot on, but let me clarify.
b = bet/pot. For example a half-pot bet = 0.5
a (alpha) is better known as the
Break-Even% or how often villain must fold for our bluff to autoprofit.
(1 - a) is better known as the
Minimum Defense Frequency, and is simply how often villain must continue to stop me from auto-profiting.