Identifying a region of a chromosome that is responsible for causing a disease (or causing some known function), but not yet identifying the exact gene.
Gene mapping is the process of identifying a specific region (or locus) along one of the 23 paired chromosomes that contains a gene with a disease-causing mutation. This is also known scientifically as "positional cloning." You can think of a region as a "neighborhood" and the gene that causes a disease as the "house" in that neighborhood. Once scientists have narrowed down the possibility to a particular locus/neighborhood on the chromosome, then they can identify the exact gene/house linked with that disease.