Strength of
Schedule.

How accurate is it?

Short answer? Some years are better than others. Below shows a single example from the 2025 season, and an interactive chart showing error across all teams by year for a given position, scoring format, and time window.
As can be seen from the Cardinals example, some positions were okay, while others were drastically wrong. You may have drafted James Conner with the expectation his schedule was incredibly easy, only for the actual schedule to be one of the hardest for RBs on the year. Vice versa with TE and Trey Mcbride.
The interactive graph shows average error, which takes all the SoS values for each team that given year and finds the error, then averages it. The max shows the single worst team's error from that year. The grey dotted line shows the absolute worst case median error score, which could be achieved if you ended up ranking them in reverse order, i.e. the easiest schedule was actually the hardest. The "random guess" line is the average error you'd expect to get if you randomly assigned some 1-32 number value to each SoS value. I won't go into the math on how that works (permutations), but the number is 10.66, so any worse than that and you would've been better off randomly guessing. You want error to be below (ideally significantly below) the random guess line to consider SoS a good metric.