This is the second of the Six Pillars of Calculus:
Close is good enough (limits)
Track the changes (derivatives)
What goes up has to stop before it comes down (maxima/minima)
The whole is the sum of the parts (integration)
The whole change is the sum of the partial changes (fundamental theorem)
One variable at a time
You probably have used the "track changes" feature on Word when
editing a document. If you know what has changed, you don't have to
re-read the whole document. The same thing goes for functions.
Instead of looking at the function itself, you can learn a lot by
studying how the function is changing.
This video gives a general overview of the idea of a derivative. Subsequent
videos will provide a step-by-step development.