In the simplest definition, tracking helps fine tune your tape's speed during playback.
Tracking on your tape is important because it helps ensure that it plays safely and smoothly. Good tracking depends precisely on the distance between certain components of the tape and VCR — we're talking micrometers here — which is why most VCRs offered tracking adjustment to correct any error in playback quality.
Your tapes will degrade over time. That's inevitable. When it comes to tracking, the magnetic media within tapes will gradually lose their charge in a process called remanence decay. But this is just one of the many ways tapes can fade over time, causing issues with their tracking abilities.