Surface film is the membrane that holds the river's surface together. Drop a leaf and you'll see it deflect before breaking through; that same tension is what an emerging insect has to push through to fly.
For trout, the film is the dinner table's most efficient feeding zone. Bugs are stuck — they can't sink, can't fly, and the fish doesn't have to expend energy chasing them. Riseforms that don't break the surface (just a soft dimple or a head-and-tail roll) are almost always fish eating in or just under the film.
To fish the film, trade your floating dry fly for an emerger pattern: a CDC emerger, a sparkle dun, or a soft-hackle wet. Pattern goal is half-on, half-through — body hanging below, wing case in the film. Use floatant only on the wing case so the body sinks. Watch the swirl, not the fly.