When you plot the mass or volume of product formed against time for a chemical reaction, you will usually get a curved graph. This curve represents how the rate of reaction changes over time. You can see this in the diagram below for two different reactions:
The reaction rate constantly changes, so you need to pick a specific point. To calculate the rate at a specific point, you need to draw a tangent to the curve and measure its slope.
The steeper the gradient of the tangent, the greater the rate of reaction. When the gradient of the tangent becomes zero, the reaction is complete and no further product is formed.