The agency’s fee is applied to the rewards that your delegation yields.
APR stands for Annual Percentage Rate, which represents the rewards rate for a whole year, applied to your staked funds.
The current APR, if you were to run validators nodes by yourself, is around 24.4%, which is estimated based on the current theoretical maximum network rewards. If a staking provider is charging a 10% fee it means the actual APR if you stake through a staking provider is around 22%.
Basically, for every 100 EGLD that you stake, you will be rewarded with another 22 EGLD yearly.
This does not take into account the rewards that could be staked as well: so-called compounded rewards.