Some schools calculate an index number as a weighted combination of an applicants LSAT and GPA. It's usually LSAT*X + GPA*Y = Index. At some places LSAT counts more, and at others GPA counts more.
EDIT: It's essentially used to sort the applicant pool. At a given school, anyone below a 3.0 index might be auto-reject, above 3.5 auto-admit, and the rest will be screened based on their application with the higher index number indicating higher admissions priority.