I am reading the paper "Relative Cycles and Chow Sheaves" due to Suslin and Voevodsky. Here we have the following definition:
Definition 2.1.2.
A morphism of schemes $p:X\rightarrow S$ is called an equidimensional morphism of dimension $r$ if the following conditions hold:
1.$p$ is a morphism of finite type.
2.The function $\dim(X/S)$ is constant and equals $r$.
3.Any irreducible component of $X$ dominates an irreducible component of $S$.
I would like to understand what the condition 3. means ... Thank you a lot!!!