Probability of profit is the model-implied chance that a position finishes profitable at expiration, computed from the chain's own implied volatilities via N(d2) at the breakeven. It states odds under the market's distribution, not a promise, and ignores the size of wins and losses.
Careful implementations evaluate POP at the breakeven using that strike's own implied volatility rather than the at-the-money number, and correct for the volatility smile's local slope. The number is only as good as its calibration: bucketing closed trades by entry POP and comparing forecast odds against realized outcomes is the audit that keeps it honest. The full mechanics are in the POP article.