Starting with Jellyfin server version 10.7.0 and supported clients, you can use Quick Connect to sign into your account without the need of a password. You need to previously be logged into a supported client, like the default Jellyfin Web Client.
Enabling Quick Connect
To use Quick Connect, the Jellyfin server admin has to enable this feature in the server dashboard.
Settings > Dashboard > General > Enable Quick Connect on this server
Using Quick Connect
To sign in to a supported client, you have to enter the Quick Connect code in your user settings.
Settings > Quick Connect
If the code is validated successfully, your new device will be signed in without entering your Jellyfin username or password on the new device.
The client will generate a 6 digit code, which you have to enter in the already signed in client in your user settings.