Click on this button when you wish to resize or relocate one of your fields. Any field you click on thereafter will be selected, and can be altered.
You can alter other characteristics besides a field's size and location. By choosing the Field Tool and doubleclicking on a field, you get an opportunity to change the field's style, its font, and other features.
To return to the browse tool you can type COMMAND-TAB (i.e. press the command and tab keys simultaneously).