When things curdle, they turn from liquid gradually to solid, forming clumps along the way. If you leave milk out of the refrigerator long enough, it will curdle.
When a liquid curdles, it forms curds, or lumpy solid masses. In some cases this is deliberate, as when you make cheese or tofu. Other times, you might accidentally let something curdle, like a custard or sauce, or a forgotten container of cream at the back of your refrigerator. Curdle was originally crudle, from crud (later curd), "any coagulated substance," or "congeal."