MDR/SRR (for Mission Definition Review/System Requirements Review) are two early review-steps in the conceptualization, design and implementation of a NASA mission. (I presume the two are sometimes carried out together, thus the catenated abbreviation.) They are carried out after the concept has been reviewed and before the design, and they on requirements. They are steps in the US Government Design Review process.
The SRR (System Requirements Review) checks the requirements for consistency with themselves and with the concept, and the MDR (Mission Definition Review) checks the requirements for consistency with the architecture as so-far specified, and with the resources presumed to be available.