Every Bitcoin and Bitcoin Cash transaction must be included on the blockchain in order to be valid. Each transaction on the blockchain needs to pay a transaction fee to the miners in order to be included in a block. When you click “send” in your wallet, you submit your transaction to the Bitcoin network. “Miners” collect, verify and bundle transactions into a block and one miner (appr. every 10 minutes) wins the right to add his/her block to the blockchain.
Miners are incentivized to pick transactions with higher fees because when they get the right to add their block, they can keep the transaction fees that belong to the transactions in that block.
If there are a lot of transactions being created on the network, fees for a transaction may increase as transactions starting to outbid each other for priority. When a lot of people want to send transactions, you have to bid higher if you want your transaction to be added in the next block(s). Remember, wallets never receive transaction fees, only the miners do.
The BTC.com wallet automatically uses a dynamic fee strategy to determine the best fee for your particular transaction. You can also always opt for the “Low fee” when you have enough time to wait for your transaction to confirm.