Valuing a CMS requires what is called a convexity adjustment. This is because the swap rate, viewed as its expected value at the time it is received, generally differs from the forward swap rate implied by today's market.
Each swap rate is set on an observation date on or before the payment date, as the level of a swap of the same tenor that starts on that date. A CMS can exchange the swap rate for a short-term floating rate or for a fixed rate.