Parralax is a design trend where differnt elements scroll at differnt speeds, give the "scene" a sense of depth and space. it often mimicks the phenomena of objects appearing to move more slowly relative to the viewer the farther away they are. For example, when driving, object near the car seem pass at a faster relative speed than opbjects that are far away, like distant mountains.
Parralax examples using rellax.js
Parralax Example 1
This is pretty cool