What is Probability of informed trading?

The probability of informed trading (PIN) of Easley, Kiefer, O’Hara and Paperman is, in their model of daily order flow, the share of trades that come from informed traders: $\mathrm{PIN}=\alpha\mu/(\alpha\mu+\varepsilon_b+\varepsilon_s)$, where an information event occurs on a day with probability $\alpha$, is bad news with probability $\delta$, brings informed orders at rate $\mu$, and uninformed buys and sells arrive at rates $\varepsilon_b$ and $\varepsilon_s$.