OK...
For every player that is in a tournament, they have a number of attributes: Buy-in cost, rank, rebuy cost, winnings, hits, points, etc.
When generating Stats, those values are taken for a set of tournaments and we arrive at final values for those attributes. We do this in different ways for different attributes, but because these are known values, the TD software knows how to arrive at a final value. For example, on the Stats tab you'll see "Hits", which is the sum of the number of hits a player made for each tournament. You also see "Average Hits", which is the average number of hits a player made. The TD can compute these because it knows what type of value it represents.
While the "Points" value displayed on the Players tab represents the amount of points a player has received in this (the currently loaded) tournament, the "Points" value displayed on the Stats tab represents the sum of the points a player has received in a set of tournaments. Again, because the TD knows this is a "points" value, it knows to add them up to arrive at a final "points" value on the Stats tab.
But there are other ways to rank players (or rate players or compare players or however you wish to phrase this). The "Score" value allows the user to make up their own way of rating a player's performance.
To do this, a new value is created for a player for each tournament in which they play. It is called "Score". When stats are generated, the "Score" (or "Tournament Score") formula is applied to each tournament in which they player, to create a "Score" value for that tournament (for that player). You don't see this value on the Stats tab, just as you don't see the "Points" value for some player for a single tournament (the "Points" value you see for a player on the Stats tab is the SUM of ALL of his "Points" values - for each tournament in which he played).
Now we've created a new value (for each player, for each tournament) called "Score". Just as each "Points" value was added together to arrive at a final "Points" value, we want to come up with a final "Score" value - in this case we distinguish it by calling it the "Overall Score". But the TD doesn't know how to do this, because any two users may want to do it differently. Therefore, there is another formula applied, called the "Overall Score" formula, that allows you (the user) to manipulate the "Score"s any way you wish. You may want to simply sum up the score values for a player, or you may want to add them up and divide them by the number of tournaments (an average), or any of a million other things.
Try switching the View mode on the Stats tab to get a better idea. In "Normal mode", you see the sums and averages, etc, of the attributes for a player. When in "Single field mode", you see only a single attribute, such as "Points", but you get to see that attribute for each tournament.
So, if playerX played in 3 tournaments, and was awarded 20, 30, and 100 points, respectively, for those tournaments, you'll see on the Stats tab (in Normal mode), 150 in the "Points" column (the sum of all points for playerX).
Switch to "Single field mode" and you'll see 3 columns, one for each tournament, and you'll see the values 20, 30, and 100 in those columns, breaking down the points that playerX received in each tournament.
You can do the same with the "Score" field, to see what your "Score" formula awards to a player for each tournament. Switch back to Normal mode and see what the Overall Score formula gives based on those Score values.