There were new base running stats added in OOTP20: UBR and BSR. You can see them if you go a player's expanded batting stats (you have to be in an individual player to see it, it doesn't show up on screens with multiple players)
I added up both stats for all of my players and UBR added up to the amount that showed up on my home screen so that looks like the metric used.
https://library.fangraphs.com/offense/ubr/
The number, +7.6, is the amount of extra runs that a player has added through the virtue of his base running. 0 is an average player.