Motion tracking is basically a process where we track a certain point or number of points in a footage in order to composite something on top of it so that the motion of the composited layer looks exactly as the motion of the footage on which we did the Motion tracking. Adobe After Effects has a built in motion tracker inside of it which you can use to track your footage. Though it is a 2 dimensional motion tracking, 3 dimensional tracking can also be done using Boujou, a third party Motion tracking software or Camera tracker, a plugin for After Effects by Foundry.