Question: Glibness and Counterspell for Bards/Warlocks

So, my DM isn't sure how to interpret the wording of Glibness. It says "when you make a Charisma check, you can replace the number you roll with a 15."

Is this the dice roll itself, such that your counterspell result is a minimum of 15+cha+Jack of All Trades? Or is it the final result of the roll after modifiers are added is just a 15?