A network fee is the fee paid to the miners of the network you are using for your cryptocurrency transaction. This fee is also known as a gas fee.
Every transaction on the blockchain requires a network fee. This is because miners use their own computers to verify and process transactions instead of relying on a central authority.
In return, miners are compensated through network fees. The Ethereum blockchain pays miners in Ether (ETH), and is called gwei for network fees. But on the Polygon network, the network fees are paid in MATIC. The network fee you pay will vary according to the network you use. Uniswap does not receive payment from network fees.
Network fees are determined by supply and demand of miners.
When there is a lot of traffic on a network (high volume of transactions), the network fee is higher. When there is low traffic on the network (low volume of transactions) the network fee is lower.
Here are some resources you can use to track network fees: