Change action in form

Please, help me.
I can’t understand in manual
http://www.campbellhoffman.com/form-manager-faq/#actions-filters

“fm_form_submission – Use this for a custom submission action. Run after a form has been submitted. The action is passed a single argument, which is an associative array with two keys:”

Where I need to insert this code? In JavaScript, PHP ? I think that in PHP, but where?

Help.