A cashable bonus is one that converts into withdrawable cash once its terms are met. After you complete the wagering requirement, the bonus and its winnings move into your real-money balance.
The alternative is a sticky, or non-cashable, bonus: you can play with it, but the bonus amount itself is removed when you withdraw, and only the winnings above it can be cashed out. A sticky bonus is worth less than a cashable one of the same size.
A related distinction is whether your own money and the bonus are kept apart. With some offers you play your deposit first and can withdraw it at any point, forfeiting only the bonus. With others they are combined and the terms apply to both. The bonus terms will say which kind you are taking.