A balance transfer is an option offered by many credit cards issuers which enables the card holder to use their available credit from one card to pay off the balances due on one or more other cards. Most balance transfer offers will come with an artificially low introductory interest rate, such as 1% or 0%, for a fixed period of time. After that time period the interest rate will rise to whatever was permitted by the terms of the offer.
However, it is a good way to make money. For example, if your credit card offers 0% balance transfer you can use the money borrowed from a bank to invest in something. It works especially well if your credit limit allows you to transfer significant amounts of money. Some people make more than a thousand dollars this way.