Question on polynomial regression under "trendline" option in charts:

I drew a line chart with my data, and then I used the following option:

1. On the line chart, click the right button of the mouse and select "Add
Trendline".
2. Choose "Polynomial" in "Trend/Regression Type".

This gave me a polynomial regression line using my data set and the exact
equation applied.

What I would like to know is how Excel created that equation. None of my
other statistics software gave me better fit than this, so I'm trying to
understand what the logic behind it is so that I can apply it to the language.

Any advice will be appreciated.
Thanks!!

Best,
JH