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 

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