You can enable or disable reCaptcha for register form from Theme Options – Advanced – reCaptcha settings. This is a useful tool to prevent registration spam.
To use this feature you must add site key and secret key which you take from this url https://www.google.com/recaptcha/intro/index.html
If you get this error
Recaptcha: ERROR for site owner: Invalid domain for site key
Make sure the site url is added here