In standard Dijkstra, you minimize the sum of edge weights. Here, you minimize the maximum edge weight encountered on the path. When you move from cell A to cell B, the effort is abs(height[A] - height[B]). The total effort of a path is the largest single effort among all steps.
Example: Path A has steps [, , ]. Maximum is . Path B has steps [, , ]. Maximum is . Path B wins, even though both have the same sum.