Here's how one of the PDFs at LSAC describes it:
The Admission Index
Some law schools combine your LSAT score and GPA to produce an index number to assist them in the admission process. An index number is provided on the LSDAS Law School Report for LSDAS-requiring law schools that choose to have one reported. You can calculate what index number will be reported for each LSAT score by using the values listed in the Admission Index Information Sheet. You should be aware that the index number reported to law schools may differ if a school changes their index formula after we have made this report available to you. You should contact each school to which you apply if you have questions about how it uses the index number in the admission process.
An admission index is produced by (1) multiplying the LSAT score by some constant (A); (2) multiplying the GPA by some constant (B); and (3) adding the sum of these two quantities to a third constant (C). In symbols, Index = [(A) x (LSAT)] + [(B) x (GPA)] + C.
The values of the constants A, B, and C, as selected by individual law schools, are listed on the Admission Index Information Sheet. The values of the constants selected by particular law schools may change from time to time at the request of the school. If you have more than one LSAT score and an average LSAT score is reported, an index number will also be reported using the average LSAT score. Not all law schools use these formulas, and those that do use index numbers do not necessarily use them in the same way. An index number calculated by LSAC is not the exclusive means by which a law school may combine data reflected on the law school report and/or other data. The absence of a law school from the Admission Index Information Sheet does not mean that some calculations are or are not made by the school receiving the law school report.