To photograph " long exposure " consists has that the camera memorizes what passes in front of the objective. The shutter remains open a few seconds or a few minutes, it depends.
In the daytime and at night.
The effect is often fooling, he allows, for example, to smooth the net of water of a waterfall, to give a vaporous effect to the sea and to standardize all the movements.
In the daytime, it is enough to screw on the objective a said filter " neutral grey " to darken the daylight to find the optimal conditions to this photographic technique.