We use tangent lines to a position vs. time graph to determine the velocity of an object.
Here are some good youtube videos for drawing the lines
Tangent Line Simulator 1 (simple) - Great simple simulation that let's you see how the tangent (green line) adjust to locations in a function. It even let's you adjust to different functions.
Tangent Line Simulator 2(complex) - This simulation allows you to adjust to a desired function. Also, you can turn off the secant line and just focus on the tangent line (purple line).