What is a buy to let stress test?

It is the lender checking that the rent would still cover the mortgage if rates went up.

They take a notional rate, higher than the one on the product, and require the rent to exceed the interest at that rate by a set percentage. A common shape is rent covering interest by 125% or 145% at a stressed rate, though the numbers differ between lenders.

Five year fixed rates are often stressed more gently than two year ones, which is why the term you pick can change how much you can borrow.