I can't remember the exact formula, but it is your opponent's win percentage and their opponent's win percentage to varying degrees.
I'll see if I can't find it in the forums before I make a fool of myself guessing at the exact calculation.
EDIT: From WIS ticket sent in two years ago:
Strength of Schedule is based on Opponents' winning percentage (OWP) and Opponents' Opponents' winning percentage (OOWP) with OWP being weighted heavier (about twice as much) more than OOWP.
10/21/2016 6:09 PM (edited)