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.
If something is changing at a constant rate, then that rate-of-change
is the same thing as the slope of the graph of that quantity.
It is also a conversion factor between changes in the input and
changes in the output. (If you're driving down the road at 60MPH, you might
say that you're 10 minutes away from town when you're really 10 miles away.
Using the speed of the car, you converted from the change in position to
the change in time.)