A contract is a bet with a fixed payout
Most event markets trade contracts that pay a fixed amount, usually one dollar, if an event happens and nothing if it does not. You buy a Yes contract if you think the event will happen and a No contract if you think it will not.
The price you pay is somewhere between zero and that dollar. A Yes contract at 30 cents costs you 30 cents and pays a dollar if you are right, so your profit is 70 cents before fees.
The price is the market's probability
Because the payout is fixed, the price reads directly as the crowd's estimate of the chance. A contract at 30 cents says the market thinks the event is roughly 30 percent likely.
That is the whole game: you only make money over time by buying when your honest estimate is higher than the price, or selling when it is lower. Being right about the outcome is not enough if you paid more than it was worth.
Fees eat small edges
Every venue charges something, whether a trading fee, a spread between the buy and sell price, or both. If you think a 30 cent contract is really worth 33 cents, a few cents of cost can take the whole edge.
Before any trade, write down the price, your estimate and the total cost of getting in and out. If the gap between your estimate and the price does not clearly beat the cost, skip it.