D Medals will be used to train your Z Warriors and get even more power by boosting your special attacks or getting new ones. If you love exploring in Dragon Ball Z Kakarot, don't worry, there will be plenty of medals scattered all over each zone. A note above the name of the zones indicates the number of medals collected out of the total number available.
If you'd rather go to the essentials and focus on the main story and not spend time exploring every nook and cranny of the map, no problem, there's a solution. As you progress through the adventure, you'll receive more notifications in the bottom right corner of the screen indicating that the Z-Encyclopedia is filling up, or that you've unlocked various rewards including valuable D Medals.
To access them just go to the menu and press Y or triangle depending on the platform you are playing on. In addition to giving access to such essential rewards, the Encyclopedia also provides some explanations about the world of Dragon Ball Z and acts as a guide if you are aiming for 100% or all trophies.