Multiply your LSAT by the first number. Write the product down. Multiply your GPA by the second number. Write that product down. Add the two results together, and either add or subtract the third number (depending on its pos or neg value), and you will end up with a single number. That number means nothing until you compare it to other people's numbers.
Do this by making the same calculations for last year's classes 75% GPA and LSAT. If your index number in equal to or higher than the number produced for the 75% GPA/LSAT, you will most likely get into that school. You can also do calcuations for the medians, and if your number is close to that produced by the medians, then you could consider yourself competitive for that school.