The behaviour is called canopy feeding. The bird stands in shallow water, brings its wings forward and down until the primaries overlap ahead of its head, and creates a closed circle of shadow perhaps thirty centimetres across, with its head inside.
Two effects are proposed. The shade removes surface glare and reflection, so the heron sees into the water clearly, an advantage in bright African sun over open shallows. The dark patch may also attract small fish, which move towards shade and cover. The bird typically holds the canopy for a few seconds, steps forward, and repeats, working an area in a series of brief tents.
Which effect dominates has not been settled experimentally. Glare reduction is the better-supported explanation, since the heron plainly hunts by sight, but the lure hypothesis is hard to rule out and both could operate at once. Several other herons use partial wing shading; the black heron closes the canopy completely.