Okay, so I assume that since this is in Chemistry you are referring to reaction rates.
Let's say that I decompose H2O2. If you graph the molarity of H2O2 as a function of time, it is a decreasing curve that is concave upwards. If I pick two points, let's say at t=1 and t=3, and measure the molarity at each, I will get two different values. If I use the standard point-slope equation, I will get an average rate, ie the general rate at which it was reacting between those two points.
But think about how the graph is curved. obviously, if I pick two points and draw a line to connect them, the slope of that line is not equal to the exact graph. That's because the graph has concavity: the slope changes constantly with respect to time. But the closer together my two times are, the more accurate my line between them will be relative to the original graph.
Nao let's say I were to gradually pick points that were closer together on the graph until there was almost no different between them. The slope i measure would be the exact rate of reaction at the time. This is an instantaneous rate; it is the rate of rxn at a single instant of time.