Two parties agree to swap interest payments on the same imaginary pile of money. Only the difference ever changes hands — the pile itself never moves.
An interest rate swap is an agreement between two parties to trade interest payments on the same notional amount, for a set number of years.
One side pays a fixed rate agreed today. The other pays a floating rate that resets periodically against a benchmark like SOFR. In practice only the difference is settled each period — the notional is just the number both legs are calculated from.
Say a company borrowed $50m at a floating rate. Rates rising would make its loan payments unpredictable — bad for budgeting five years out.
Rather than refinance the whole loan, it swaps: pay fixed, receive floating. The floating leg it receives roughly cancels the floating rate it owes on the loan, so its net cost becomes fixed — without ever touching the original loan.
Almost nobody who issues debt wants to sit with the exact rate risk that debt happens to carry. A government that just sold floating-rate bonds would rather know what next year's interest bill looks like. A pension fund would rather its liabilities stop swinging every time a central bank meets. So a swap gets layered on top, quietly doing the rate management off to the side.
The loan or bond underneath never has to be touched, refinanced or reissued — which is really the whole point. It's also why swaps trade in such enormous volume: the debt itself moves slowly, but everyone's opinion on where rates are going changes constantly, and a swap is the fastest way to act on that opinion.