The letter ID on a car title is an ID number that comes in the form of a letter that is mailed to you from the DMV.
When you're trying to register a vehicle online for most states DMV websites then you'll be asked usually for a Letter ID.
If you haven't gotten the letter ID in the mail with the title to the vehicle then you need to call the DMV and request the letter ID which is sent by mail.
This is how it is here when I need to have a Letter ID to register a vehicle.
In some places it may differ.
The Letter ID I thought was at first the VIN number but turns out it's an ID number that comes in the mail on a letter that you then enter on the DMV website.
Letter ID usually comes in the format L123456789.