A payout is what you will receive at the conclusion of a trade, this may be at the option expiry or in the case of early closure. A payout is expressed as a percentage or multiple.
For example, we invest $100 into a trade on the USD/JPY with a 185% (or 1.85) payout. At expiry, the trade is in-the-money, meaning we receive $185.
The original $100 invested + $85 of profit.