/System/Library/Frameworks/CoreServices.framework/Versions/A/Frameworks/Metadata.framework/Versions/A/Support/mds_stores mds_stores is an internal process that is used by Mac's Spotlight application. MDS or metadata server is a function that is used to keep track and record important information about data that is used to search for files on the system. mds_stores compiles and uses this metadata to speed up the search process used by Spotlight. Without mds_stores, Spotlight would have to start searching at the beginning of the hard drive and go through each file one by one. This could take an extremely long time.
The binary is a part of CoreServices public framework.