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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 are followed by reviews of subsequent iterations of the design, each covering additional detail. They are steps in the US Government Design Review process, for example, used for major military projects.
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.
A subsequent step in the US Design Review process (and a step also common in design review processes of other organizations) is the PDR for Preliminary Design Review, to assure that the preliminary design meets to previously-defined requirements.