Slang for Life Without Parole (or Death) Sentence. All other sentences have some type of number on it (as in timeline) as you would have a release date. Letters mean you don’t have a release date and you will die in prison or jail.
Referenced by Larry Lawton a well known after prison YouTuber.
Jake: I got life without parole.
Larry: Damn you got letters! You never getting out!
A letter is a logo of a specific sound. For example the letter A is a logo of the sound you make when you say ‘A’ . The letter A can be said in many different ways, however it’s written the same (aside from capital letters and lower case) .