Recaptcha it's a free service offered by Google, the function of recaptcha is to block all bots from visiting your website. Captcha it's a very common thing these days, and definitely it's a must for your website, specially if you're worried about suspicious visits that comes from bot softwares.
The best thing about recaptcha it's not that they only will block bot softwares, but it also will help you to get accurate results about your website visitors. Recaptcha it can be installed very easily to your website, basically you need only to put your website url there and click the button register, then you'll need to paste the snippet of Recaptcha to your html template, and that's it.
When you need recaptcha? For example, if you have a contact form to your website, and want to keep it safe from abusers then recaptcha it's the right thing to use. If you're a wordpress user don't worry you also can install it for free, simply search for google recaptcha and you'll find it there.