The Regression option is useful when you need to show a trend in your data - typically when working with time based data. The Regression option will provide the best straight line through all of your data points as in the example below.
The Regression option is a toggle that you can enable/disable by clicking it.
Notice: If your data suits hyperbolic (up to 4th polynomium) regression lines, these options will appear as you use the toggle option. In the example below, the data apparently supported a 2nd polynomium regression line as well.
Furthermore, if you click the object's information "i", you will be able to see the exact formula that is used to produce the line: