Reagents are items which help you to train your characters' skills and make them more effective. They can be obtained as rewards or by disenchanting a hero. When a hero is disenchanted it is consumed and can no longer be used in battle.
To disenchant a hero, click on the "Disenchant" button when viewing the character card or when viewing the new heroes you just earned.
You can view all of your heroes and reagents by tapping on "Collection" from the main menu. There you can tap the arrow in the upper right to view your inventory of reagents.