Account private key
A private key, in the context of cryptocurrencies, is a secret number that allows coins to be spent. Every wallet contains one or more private keys. The private keys are mathematically related to all addresses generated for the wallet.
Trezor and Account private key
In the context of Trezor device, the only relevant information is that the account private key is mathematically derived from the recovery seed using BIP39 standard. The account private key is used to generate all private keys, public keys and addresses for specific account. It is essential to store the recovery seed in a safe place - without it, all the private keys and funds are irreversibly lost.
Because the private key is the "ticket" that allows someone to spend coins, it is important that it is kept secure. Trezor allows the users to store the private key safely - it never leaves the device, and all transactions are signed in Trezor.