In both postal and philatelic environments, "franking" is defined as the act of affixing postage to a letter or parcel prior to mailing it. Before the introduction of U.S. postage stamps, an individual would take mail to a post office where the rate was determined by a postman. After the individual paid the postage, the word "paid" was written in the upper right hand corner and the item was accepted for mailing. If you have a quantity of "unfranked" stamps, meaning they have not been used for mailing, using them is quite simple.
Visit any post office.
Present your letters or parcels to the postman. They will weigh them and determine the correct amount of postage.
Affix your stamps to the letters or parcels and hand them back to the postman. You have properly franked your mail, and your stamps have been used.
To facilitate mailings to their constituents, members of Congress have been granted the "Franking Privilege," This permits them to to send official mail using their signature instead of stamps.