The rs number is an accession number used by researchers and databases to refer to specific SNPs. It stands for Reference SNP cluster ID.
A SNP is a location in the genome that is known to vary between individuals. When researchers identify a SNP, they send a report (which includes the sequence immediately surrounding the SNP) to the dbSNP database. If overlapping reports are sent in, they are merged into the same, non-redundant Reference SNP cluster, which is assigned a unique rsid.
Genome-wide association studies linking SNPs to traits or conditions usually report their results by rsid. The rsid numbers for SNPs in Health and Traits reports can be found in the Scientific Details section.