Honeypot

A Honeypot field is present by default in every form as a part of our anti-spam system.

It is basically a fake input field that is only visible to bots, and not to humans. This is achieved with some clever CSS rules. Genuine web browsers respect the CSS, while bots usually just parse the raw HTML of a page to identify the fields of the form, and they tend to fill out all the fields they find there.

If the honeypot contains some data, it is safe to assume that it’s a bot since humans can’t see it.

However, next-generation spam bots do not fall into honeypot easily. By their nature honeypots fields must be "non-mandatory", because humans has no chance to fill out a field they don't even see.
Well, modern spam bots are quite conservative, and they just skip any field which is not mandatory, just to avoid to fall into a honeypot.