The ‘reCAPTCHA’ form element is a spam / bot blocker.  It can be thought of as a ‘human test’ – it distinguishes between human beings and spam bots. The Google reCAPTCHA shows an image of some distorted text, and asks you to enter the text.

To use a reCAPTCHA, simply insert one into your form – though you have to enter the reCAPTCHA API keys in the ‘Settings’ page under ‘Forms’ in WordPress.