Every stake pool operator receives a minimum fee for every block that is produced. This fee covers the costs of operating a stake pool which can include computers, servers, websites and software all required to run a stake pool. The minimum fee is 340ADA, this number is taken from the total rewards prior to rewards being delegated to each individual staker.
The variable fee is charged by some stake pool operators to help cover additional costs involved with running a stake pool. Our fee is currently at 0% and if this ever changes we will let you know why and how long it will be for.