The left table of the dialog contains the data fields currently in the database. You can select which fields you wish to be active by setting the check marks accordingly. You must have at least one checked, unless you have added one or more fields in the New Fields box.
Note that the unchecked fields are not removed from the database, so no data will be lost in unchecking them. They can be made active at any time in the future. However, bear in mind that no data will be entered into those inactive fields in the interim.
The Reset button will set the checks back to the original setting when the dialog was opened, and the Clear button will uncheck all boxes.