Gas fees refer to network fee, which is the amount payable to validators that are expending computing power to validate new transactions on the blockchain.
Gas prices increase with rise in transaction demands. This is why Ethereum tends to have very high gas fees. As more dApps are built on the network, more transactions are requested, causing validators to prioritize transactions that pay higher network fees.
Typically, when paying gas fees, users are given the option of choosing among slow, medium and high gas fee. The higher the gas fee the user pays, the faster their transaction will go through. If the gas fee paid is too low, the transaction can take days or even weeks to go through. Sometimes, it could remain in constant limbo. So, it is very important that you choose your gas fee wisely.