Publisher - is a general traffic source identifier that gathers and sends leads to the system and gets paid for it. It could be an individual publisher or a partner network that works on attracting clients.
Publishers use various methods to gather the leads by processing calls (phone leads), adding the form on their website (webform leads), etc.
For example, Publisher adds a Form with a loan offer on his website. The potential client who wants to loan a specific amount fills in this exact form. The lead (entered data) is sold to the Buyer (Bank) to obtain a new customer.
Read how to create a new Publisher in the Knowledge Base.