The idea of Adjacent Possible is that most innovation/discoveries happen on the edge of current progress. And previous innovation/discoveries determine what is possible to discover now.
New innovations kinda follow from what is available now and are obvious for those looking in the right direction. This explains why many theorems/discoveries are discovered simultaneously and independently by multiple scientists (and that happens often).
Adjacent Possible is the region just beyond the current cutting edge—to get to it, you need to be on the cutting edge.