In sales, marketing and lead generation, a lead magnet refers to an item (such as a PDF report, a video or a sample piece of software) designed to attract leads.
The "lead" is the information people give in exchange for the lead magnet, which could be just an email address, or perhaps more details such as name, telephone number, job title, company name and so on. You can use this information to follow up on leads, potentially turning people into customers or clients.
As well as attracting leads, in preselling you can use a lead magnet to move potential customers closer to the point where they're ready to buy. This is one of the main purposes of a presell report, a special type of lead magnet used in the Presell System.