The Goal page is the first page of the Fitting Function Builder. You can choose to create or edit a User-defined fitting function in this page. Depending upon the chosen goal, subsequent pages of the Builder will display some minor differences.
The Fitting Function Library App allows you to look for additional fitting functions, available for download from the OriginLab website. You can (1) download the App from the OriginLab website or (2) you can install it directly by pressing F10, searching on "function" and clicking the Download and Install button.