A balance transfer is a way to move your balance to a different card, which may or may not have a different interest rate (for example, from a 19.9% interest rate card to a 10.9% one). There are 2 main reasons for transferring a credit card balance.
To pay less interest and pay off your balance faster
When you transfer your balance to a lower interest card, you can pay it off faster since more of your money goes towards the principal. For the same payment amount, your balance will go down faster on a lower interest card.
To consolidate your payments
If you transfer all your credit card debt to a single card, you only have to make one monthly payment, not several. This makes it easier to manage your finances, with fewer risks of falling behind.