A balance transfer involves moving debt from one or more existing credit cards to a new credit card, typically offered by a different financial institution. The main appeal of a balance transfer is that the new credit card often comes with a promotional 0% or very low introductory interest rate for a set period (e.g., 6 to 24 months). Just like telcos give new port-in discounts unavailable to existing users, balance transfers at very attractive rates are used by banks to attract high spenders.
It looks very similar to debt consolidation, where a single lender takes over all your personal loans or business loans, except that in this case, his or her advantage is not about attracting a high spender but rather about not having to lend multiple small amounts to multiple individuals or companies, which increases his/her operational costs, such as underwriting costs.