Pigeon is gambling slang for an inexperienced or naive player who is easy to beat, the kind of opponent stronger players hope to find at their table. The word is unflattering, close in meaning to fish at the poker table or mark in the language of con artists, and it suggests someone who can be talked into poor bets or drawn into games they do not understand.
Calling a fellow player a pigeon is considered rude, so the word belongs in conversation rather than at the table. For operators, the idea behind it is a responsibility: newer players benefit from clear rules, demo modes, sensible default limits and readable game information, and in peer-to-peer games such as poker, fair-play controls protect less experienced customers from collusion and predatory play.