Secondary markets refer to the trading of a security that already exists. Also called seasoned, these securities most likely have been trading on an exchange throughout their existence. Secondary markets for every asset are much larger than their primary counterparts.
For stocks the biggest secondary market exists through the major exchanges in the world. Examples are the New York Stock Exchange, the London Stock Exchange and the NASDAQ. Secondary markets in stocks also exist in inventories that broker dealers buy to mark up and sell.
The bond secondary market refers to any bond being offered that is not an original issue from the issuer. Since their is no physical or common exchange for bonds, the trades take place all over the world, usually from private inventories.