RMD stands for required minimum distribution. It's the minimum amount of money you're required to take out of your retirement plan once you turn 72.
RMD rules apply to all employer-sponsored retirement plans, such as pensions, profit-sharing, 401(k), 403(b), and 457(b) plans, as well as Traditional IRAs and IRA-based plans, such as SEPs, SARSEPs, and SIMPLE IRAs. However, RMDs aren't required for Roth IRAs while the owner is still alive.
Your 1099-R issuer is required to follow RMD rules and regulations, so if you received a distribution and you're at least 72 years young, you can be almost certain you received an RMD. Check with your plan administrator if you're still not sure.