reCAPTCHA V2 is extremely important these days to verify bots aren’t trying to take advantage of your site.
First, you need to get a site key (this is a public key, used in the HTML and necessary to render it) and a secret key (this is a private key used by your site to communicate with Google server.
You can get these keys from https://www.google.com/recaptcha/intro/.
Site key is used to display the reCAPTCHA widget
Secret key helps authorizes communication between your site and the reCAPTCHA server.