Can someone explain what this is and how it's calculated please.

I'm unsure about the calculation of it myself, but basically its a measure of how strong your opponents are.

As an example, you have a 3-round tournament, and lets say you win round 1. If the person you defeated in round 1 goes on to win their round 2 and round 3 matches, your own strength of schedule would be higher than, say, had said opponent then gone on to instead lose round 2 & 3.

Basically the better your opponents do in other rounds, the higher your strength of schedule.

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Strength of Schedule: A player’s strength of schedule is calculated by dividing each opponent’s total tournament points by the number of rounds that opponent has played, adding the results of each opponent played, and then dividing that total by the number of opponents the player has played. The player with the highest strength of schedule is ranked above all other players in the group not yet ranked. The player with the second-highest strength of schedule is ranked second among all players in the group not yet ranked, and so on.

From the Tournament Rules:

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Strength of Schedule: A player’s strength of schedule is calculated by dividing each opponent’s total tournament points by the number of rounds that opponent has played, adding the results of each opponent played, and then dividing that total by the number of opponents the player has played. The player with the highest strength of schedule is ranked above all other players in the group not yet ranked. The player with the second-highest strength of schedule is ranked second among all players in the group not yet ranked, and so on.

An easier way to explain it is that your Strength of Schedule (or SoS) is the average of the wins-per-round-played of your opponents. An opponent who played 3 rounds and won 2 of them has a wins-per-round of 2/3. Add up all your opponents' wins-per-round numbers and divide by the total number of opponents you had. (Part of the funky wording is to account correctly for players who drop from the tournament midway through.)

So, if you played a 3-round tournament, and your opponents scored 1 win, 2 wins, and 2 wins respectively (and everyone played all three rounds), you'd have an SoS of (1/3 + 2/3 + 2/3 ) / 3 = 5/9 or about .55

SoS is currently used as a secondary tiebreaker after MoV. It used to be a primary tiebreaker in an older version of the tournament rules, but it's subject to distortions due to players dropping after they lose a couple rounds early in the tournament, then dropping instead of going on to win some of their later, easier match-ups. Thus, MoV is considered preferable because it's primarily based on your own game results.

I'd say the simplest answer is that it is the combined win/loss record of all of your opponents. The better your opponent's net W/L record is the better your SoS will be.

Thanks it is much clearer now.