A CAPTCHA is a Turing test that allows you to differentiate between humans and bots. It is easy for humans to solve, but hard for spam bots and other malicious software to figure out.
The Google v2 Invisible reCAPTCHA will not bother your users unless they consider them to be spam. It will appear in the bottom right corner of the page, and it will only launch the challenge to users with suspicious activity.
When a user clicks the submit button on the form, it will assess a risk score and, if necessary, display an image reCAPTCHA challenge to solve. The form will not be sent until the user solves it. Humans will pass the challenge and submit the form, while bots will get stuck in it.
It is especially useful in flows with associated costs, such as SMS, coupons, etc. to block spam and automated brute force attacks, and it wouldn't affect their conversion rate.