An invisible field used to detect bots since it does not accepts any value. If gets filled, it will return an error (like an email field when receives an invalid email address).
Since is an invisible field humans will not fill it but bots read your website code trying to fill all fields and commonly they fall in this kind of trick (for that is called honeypot!).
BBForms adds a honeypot field to all forms with the option
disable_honeypot set to no. You can turn it to yes to disable this default honeypot (the ones you placed in the form will continue working).