The Equation Selector tool helps you choose a function to work with. Functions are used to manipulate your data, similar to how equations and functions work in Microsoft Excel.
For a step-by-step example of using functions, see the About Equations article.
If you don't already have a function in your "select" then clicking on the Equation button will open the Equation Selector.
The Equation is accessible from the Tag Editor.
It is also accessible from the Ribbon.
Equation Selector Window
Use the equation selector window to choose a function. You can filter which ones show by setting a category.
When you click on a function, it will give you more information on that function.
Once you have selected a function, click Ok to continue to the Equation Editor.