The link says to add in the Tag library descriptor and reference in jsp.
"In order to define an EL function, a <function> element must be added to an existing or new TLD file that is in the scope of your application and declared within the referencing JSP page. For our example, if we wanted to name the EL function the same name as the static method (though they do not need to be the same), we could add"
Please go through the information provided in the link again.
Once you've created a TLD, it can be used not only to hold EL functions, but custom tags, and tag file declarations. It's a very useful element to have in your project to define project-specific JSP elements.