A honeypot is a technique used to reduce comment spam, and other forms of spam submitted via online forms. The concept is to attract bots to the honey, so your form can recognize which submissions come from automated bots, and which are from humans.
The most common honeypot technique is to add a hidden field to your form that humans looking at your site in a browser do not see, but robots looking only at your code will see. For exmaple, if you have a hidden field called “email-address-hidden”, the robot will complete it with an email address. But if that field is hidden with the use of CSS or Javascript, it will not display in a browser. Then your form simply trashes all submissions that have the “email-address-hidden” field filled out.
The user benefits from not having to deal with a confusing captcha image, while keeping your site nearly spam-free.
Many form plugins give you the option of using a honeypot. Gravity Forms is one of my favorites.
P.S. In the larger world of server security, a honeypot is a server or site configured to attract nefarious traffic, deflecting it from the intended target.