A clumping, usually of proteins, and usually in milk or milk products. When you are making cheese, of course, you WANT milk to curdle...that's where the curds come from. But if you are making cream of tomato soup, curdling is a "bad" thing. It makes the soup look odd, although it is still nutritious.
Acid (tomatoes, lemon juice) and/or excess salt can cause milk to curdle.