From the introduction:
Have you ever had an adventure planned out, but for some reason or other, the PCs fail to find the vital clue that will allow them to proceed? Maybe they have to talk to an important NPC, find the right piece of revelatory information, or discover a map. But for reasons that make sense to your players, they try to kill the important NPC, disbelieve the revelatory information, or discard the map as unimportant.
Rather than toss the whole adventure out, the GM needs another way to get that clue to the PCs. The adventures in Explorer’s Keys are designed to do this, and in a way that requires essentially no preparation on your part.