This is a form of financing offered by some high street banks and a number of specialist finance companies.
In essence they lend you money, secured against the money due to you from clients. Most will lend you up to 85% of the value of the invoice as soon as you send it to your customer, and then paying the rest (minus their fee) when the customer pays the invoice.
If cashflow is tight, or if clients demand long payment terms, this can be a good way to get money flowing into the agency more smoothly. But it does mean giving up a non-trivial percentage of each invoice, which the finance company takes as its fee.
There are two levels of service, factoring and invoice discounting.