A YES or NO price represents the market's current estimate of that outcome's probability.
Prices change when traders place and fill orders. They are estimates, not guarantees.
Example: You buy 10 YES shares at $0.40. The shares cost $4.00 before fees. If YES wins, they redeem for $10.00. If NO wins, they become worth $0.
The displayed probability may be based on the midpoint between the best available buy and sell prices.
When buying, you normally pay the lowest available ask. When selling, you normally receive the highest available bid. A wide spread or low liquidity can make your actual price different from the displayed probability.