The underwater world is filled with edges. Physical changes in the underwater environment that focuses fish movement and behavior.

The shoreline is in effect the irregular edge of the lake where the domain of fish begins. Visible to both man above, and fish beneath the surface.

The bottom of the lake forms an inconsistent edge featuring complex variations in depth, bottom content, and weed growth. Clearly visible to the fish, but challenging for anglers to interpret without the aid of modern electronics.

Drop offs to deep water form edges against which fish congregate, travel, and feed. But probably the most misunderstood edge is the surface. Viewed from above, the surface floats your boat. Yet from the fishes perspective below, it forms a ceiling against which they can trap baitfish.
The better you can understand and visualize these edges, the better you can grasp how fish relate to them — and how and where to catch them.