reCAPTCHA is a free anti-spam service of Google that protects your website from spam and abuse. Once you register your website for Google reCAPTCHA, you will get a site and secret key. So, to activate the reCAPTCHA, you need to set both site and secret keys.
(1) Site key: Set the site key here that you have got from google reCAPTCHA.
(2) Secret key: Set the secret key here that you have got from google reCAPTCHA.