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 

Use Recaptcha V2 to create the credentials.

If you get this error

Recaptcha: ERROR for site owner: Invalid domain for site key

Make sure the site url is added here