From http://www.nwea.org/assessments/map.asp

What is the RIT scale?
The scale NWEA uses to measure a student's progress is called the RIT scale, short for Rasch Unit. The RIT scale is an equal-interval scale much like centimeters on a meterstick. It is used to chart a student's academic growth from year to year.
If you would like more information on the RIT scale, refer to the following materials:
Hambleto, R.K. , Swaminathan, H., & Rogers, J. (1991). Fundamentals of Item Response Theory. Newbury Park: Sage.

Ingebo, G. (1997). Probability in the Measure of Achievement. Chicago: MESA Press

Wright, B.D. & Stone, M. (1979). Best Test Design. Chicago: MESA Press.

Visit http://edres.org/irt/ for introductory material about the item response theory and the Rasch model.