In cryptocurrencies, a transaction fee (or shortly fee) is a payment to the miners for adding a transaction into the blockchain. When a transaction has been included in the blockchain, it is considered confirmed. The amount of fee sent with the transaction determines the confirmation time.
The Trezor Wallet allows you to determine what amount of fee to send with a transaction: low, economy, normal or high. The amount of the fee provided determines how fast a transaction is confirmed. It is also possible to set a custom fee. However, it is important to make sure the fee will be sufficient for the confirmation time you are trying to achieve; sending a transaction with a fee that is too low might mean that the transaction never gets confirmed.