Motion tracking is a bit like choreographing a group of dancers to move in the right direction at the right time, without tripping over one another or kicking each other in the teeth.
It's the unsung instructor standing in the wings, who often brings it all together even though they don't get the applause at the end.
Using motion tracking, you can follow a filmed movement from frame to frame and apply that movement to another layer or object, using the data collected. This gives the effect of unified matching movement that's seamlessly orchestrated with the footage being tracked. This is useful in compositing digital elements into final shots and results in text, images, animations, effects or other video precisely matching the original moving footage.