Rather than wait 30, 60 or 90 days to get paid for a service, a service provider sells their invoice to another company (called a factor). The factor pays a large percentage (e.g., 90%) of their invoice immediately. When the invoice eventually gets paid (e.g., 60 days later), the payment goes to the factor. The factor then pays the service provider rest of the invoice (minus a small fee).
Instead of waiting to get paid, a company can get paid immediately (by a factor) for a small fee.
To learn more about factoring, visit the factoring Wikipedia page.