Open URL is the script step that will open the default browser to the specified web page. If the "email" is a "mail" type URL, they will open the needed email program.
But with latest versions of FileMaker, you also have the Send Mail option for sending Email from FileMaker so you may want to also research that option.
How do I apply this to the actual field?
Open URL references a calculation. This calculation expression can be a reference to the field like any other calculation. For examples of Open URL, you can take a look at my Known Bugs List database. There are several fields with URL's on the Bug Report layout with a button to the right that opens the specified page in the system's default web browser.