Long Time Exposure is a special technique in what (as the name tells) it is about long exposures. When you make long exposures, you have different conditions for the light what drops at your photos.
In the night it is clear, that there is less light, and to catch it you use more time.
During the day you can make long exposure photos as well, if you reduce the light. For that purpose there are special filters, ND Filters. You put that at your camera, and the light is reduced. I do not want to go into technical details here, now, it is about photos.
The Effect you get, when you, for example, make a long exposure photo of , is that the water does not show waves anymore, but becomes smooth and (some say) romantic.
It is not an easy discipline, I made my first attempts at this sunday morning in September.