Polymarket and Kalshi are prediction markets: exchanges where people buy and sell yes-or-no contracts on real events, including player stats. You're trading with other people, not betting against a sportsbook.
A contract pays $1 if it happens and nothing if it doesn't, so its price, from 1¢ to 99¢, reads like a percentage. If "over 4.5 receptions" trades at 55¢, the market puts it at about 55%. Prices move as people trade, and they react fast to injuries, lineup news and role changes.
Because it's an exchange, you can usually sell a contract before the game finishes, at whatever the market pays then. That lets you lock in a gain or cut a loss.
The same prop often trades at different prices on the two exchanges. A cheaper price means a bigger payout for the same pick, so every board shows both prices and marks the cheaper one. The model's edge is measured against that cheaper price.
Each site sets its own terms. Availability, age requirements and rules vary by location, so check each site before you sign up.