An RD/RDN is someone that has completed at least a BS in Dietetics, one year of supervised work, and passed the registration exam to become licensed. An RD/RDN must also complete continuing education credits regularly to maintain their license. Some states also require receiving a state license in order to practice in that state.
Those registered can either use the credentials RD or RDN. Traditionally, it was RD but due to confusion among the public between a dietitian and a nutritionist, we are allowed to use either credentials.
For more info on what an RDN can do for you, check out this article from the Academy of Nutrition and Dietetics: What can an RDN do for you?