Juris Doctor or Doctor of Jurisprudence [0], a kind of law degree
In this case, it refers to the literal letters written in the signature of a letter, which presumably indicate that whoever wrote it studied a bunch to get into school, paid a bunch to go to school, graduated with that degree, and should be respected, because they may have the knowledge, willingness, and resources to take the words in the letter further, if they want to, given that they have already somewhat attached themselves to the cause in the form of the letter.
