This is probably a very stupid question. I'm sorry.

Let $D$ be a simple normal crossings divisor on some smooth projective variety $D$. By this I mean that the irreducible components $D_i$ are smooth and all possible intersections $\bigcap D_{i_1} \cap \cdots \cap D_{i_k}$ are transversal.

I don't understand what people mean by the singular locus of $D$. In view of the definition, I would say $D$ is smooth. Could anybody help me clarify this point?