A franking machine is something that franks (indicates that the letter has been paid for) outgoing mail with the price and date. On the franking machine you usually weigh the letter to give it a price, then it gets automatically fed through and franked. To do this you select whether the letter is small or large, weigh the letter and then select whether you want it sent out as first class or second. This then gives the letter an appropriate price to send out to which you then feed it through the franking machine.
Sometimes, letters are too big to fit through the franking machines so to overcome this you weigh the letter like you usually would and then just send a sticky piece of paper through so you can then stick that onto the letter after. You have to always make sure there is sufficient amount of credit on the franking machine otherwise the items won’t be franked.