Google reCAPTCHA is a free service that must be added to protect your site from spam and abuse. It uses advanced risk analysis techniques to tell humans and bots apart.
Use this link to get your Site and Secret Key:
The Label field is optional.
In the Domains field, add your registered domain name. Click Register. A new page will open revealing the Site and Secret Keys.
Copy and Paste these into the fields under Google ReCAPTCHA Key in your Site Settings > Miscellaneous. Click Save.