deltaX is a small displacement along the x-axis.

f'(a) is the slope of the function at x=a.

f'(a)*deltaX can be understood to be the displacement of the function given the small displacement of x.

f(a) is the value of the function at x = a.

f(a)*deltaX can be understood to be, with a measurable error, the area under the curve - a tiny rectangle with base deltaX and height f(a).

It is when deltaX shinks that things get REALLY interesting.