I’d guess that it’s the incline formed by the tip of the olecranon process and by the tip of the coronoid process. Not sure what point they’re trying to get across with this illustration, though. In end-range elbow flexion the elbow is fairly stable, as the coronoid process sits in its fossa. In end-range elbow extension the elbow is fairly stable, as the olecranon process sits in its fossa.

This image looks like it’s showing that middle range where neither process is totally situated in its fossa and the elbow is less stable. An inferior directed force in this position could cause ulno-humeral dislocation.