A PIN (or personal identification number) is a numeric password used in the process of authenticating a user to a system.
Using PIN with Trezor 
In Trezor security design, the PIN plays a critical role because it protects the device against physical attacks. Read more about PIN use in Trezor blog here and here. For more information about PIN security, see also Security best practices.
Entering PIN 
Trezor employs an original method of entering PIN. Trezor One uses a blind matrix (see this video). Trezor Model T uses its own touchscreen. Entering the PIN like this is both convenient and highly secure. Therefore, unlike a PIN entered on a PC or a smartphone, a PIN entered on Trezor devices cannot be compromised by malware. For more information, see User manual:Entering PIN.
To prevent brute-force attacks, after every failed attempt, Trezor enforces a delay before it lets you try a different PIN again. This delay increases by the factor of two each time. Moreover,
Setting up PIN 
The PIN is set during the device setup. The PIN can be up to nine digits long and it is recommended to have a PIN that is four or more digits long without repetitions.
Changing PIN 
Resetting lost or forgotten PIN 
In case you have lost or forgotten your PIN, your only option is to wipe your device (see Wiping the memory of a locked, PIN-protected device) and recover it with your recovery seed. Once the recovery is complete, you can set a new PIN. In case you lost your recovery seed (see User manual:Recovery), there is not much you can do (see Emergency situations).
See also: Passphrase