A plain-vanilla interest rate swap exchanges a stream of fixed-rate payments for a stream of floating-rate payments (now typically referencing SOFR in USD or SONIA in GBP) on the same notional principal. No principal changes hands, only the net interest difference on each payment date. At inception the par swap rate is chosen so the present value of the fixed leg equals the present value of the floating leg, making the swap worth roughly zero to both sides on day one. After that, every move in the yield curve pushes the swap's mark-to-market into the money for one party and out of the money for the other, which is exactly what the post-trade machinery is built to manage.