The RPI (Rating Percentage Index) is a measure of strength of schedule, and how a team does against that schedule. It does not consider margin of victory. It is used by the WCAA as one of the factors in deciding which teams to invite to the National Tournament and where to seed them.
The basic formula is 25% team winning percentage (WP), 50% opponents' average winning percentage (OWP), and 25% opponents' opponents' average winning percentage (OOWP). When calculating winning percentage (WP), the RPI weights a road win as 1.4 x a normal win, a home loss as 1.4 x a normal loss, a road loss as 0.6 x a normal loss and a home win as 0.6 x a normal win - neutral site games are not adjusted (weighted at 1.0).