In simple terms, a company will send out an invoice to a customer, who will have pre-agreed payment terms. These are usually 30, 60, 90 and 120 day payment terms. A finance company (the factor) will look at the strength of the customers, the borrower and further possible security offered.
Invoice factoring is an agreement to assign your accounts receivable (A/R) to a factoring company. So the letter communicates that a third party (factoring company) is managing and collecting your A/R.