In the context of Bitcoin, an input is a reference to an output from a previous transaction. Multiple inputs are often listed in a transaction. An input can only be spent as a whole. If a payment is smaller than the input, the remaining change is sent back to the user's change address. For more information, see this external article.
To make the user experience better and easier, Trezor Wallet calculates all the inputs and outputs and displays only the final balances and transaction amounts. For more information about Trezor Wallet and transactions, see Transaction details.