Answer
The instantaneous velocity is given by
$$v(t)=\lim_{h\to0}\frac{f(t+h)-f(t)}{h}.$$
It can be interpreted as the slope of the tangent line of the graph of $f$ at the point $(t,f(t))$.
Work Step by Step
The instantaneous velocity says how much the position changes per unit interval of time in the limit when the length of this interval tends to zero which is
$$v(t)=\lim_{h\to0}\frac{f(t+h)-f(t)}{h}.$$
Here the length we divided the small change in position $f(t+h)-f(t)$ with the small time interval $h$. This expression matches the definition of the slope of the tangent line of the graph of $f$ at the point $(t,f(t))$ so in terms of the graph of $f$, we interpret the instantaneous velocity as the slope of the tangent.
