What is interest only and why do landlords use it?

You pay only the interest each month and the balance stays where it is. At the end of the term the whole loan is still owed.

Most buy to let lending is arranged this way. It keeps the monthly payment lower, which helps the rent cover the mortgage, and the interest is what gets the tax treatment rather than the capital.

The trade off is that nothing is being paid down. You need a plan for the end of the term, whether that is selling, refinancing or repaying from somewhere else, and it is worth having that plan before you need it rather than after.