The “adjacent possible” is the collection of all currently possible innovations if we Invent by combining things in new ways.
Imagine the current state of the world as a room with four walls, each one with a locked door. If we find the right combination we can unlock a door into an adjacent room.
That room again has four doors, three of them locked. If we can find another combination, we can make it to the next adjacent room. The adjacent possible is all the rooms we can theoretically reach by opening one door.