An unconfirmed transaction is a transaction that has not yet been included in the blockchain. Until a transaction has been included in the blockchain, it cannot be considered final. The amount of fee sent with a transaction has an effect on the confirmation time. Unconfirmed transactions are considered unsafe, as the transaction may never be confirmed or replaced with another transaction. Therefore, it is advised to wait until a transaction has been confirmed before accepting it as payment.
The Trezor Wallet will show a transaction as unconfirmed if it has not yet been included in the blockchain. After a transaction has been confirmed, it will typically receive multiple confirmations soon after. At this point it is safe to accept the transaction as payment.