I assume you are referring to box 12 on a W-2. Each entry in box 12 is a one-letter or two-letter code and an amount. The FAQ at the following link lists the meanings of the codes.
https://ttlc.intuit.com/questions/1899457-what-do-all-the-letter-codes-in-box-12-of-my-w-2-mean
If there is something you still don't understand after looking at the FAQ, add a comment here telling us what codes you have in box 12 and what your question is.
