I heard the other day the commentator said a player is in "scoring position" after running to the second base.
What is a "scoring position" in baseball/MLB?
It is exactly being at second or third base. The point is that it is (usually) enough to hit a single to score a run by that player, which is not true for players at first base, or home.
For a base hit it typically has to get out of the infield so a runner on second can score unless they had to hold for possible caught in the air.
On third runner can typically score on even a caught ball (sacrifice fly).
From first not going to score without error(s).