The strength of schedule (SOS) rating is very closely related to the RPI rating.  This will explain just how close.

The SOS rating is a weighted calculation that combines 2/3 of your opponents' winning percentage and 1/3 of your opponents' opponents' winning percentage.  For the RPI calculation, you use this same 2-to-1 ratio and combine it with a team's actual record against their opponents.  The final RPI ratio is 1 part your team's record, 2 parts your opponents' record, and 1 part your opponents' opponents' record.

Essentially, the SOS rating determines how strong your opponents are and the RPI rating factors in how you fared against those opponents.  For details on how to calculate these figures, see the article that explains the RPI ratings page.