Let $f:X \longrightarrow Y$ be a morphism such that any irreducible component of $X$ dominates $Y$. $(1)$
My question is:
a) what does it mean any irreducible component of $X$ dominates $Y$?
b) if $f:X \longrightarrow Y$ is a dominant morphism, it's true $(1)$?
a) It just means that if you restrict the map $f$ to an irreducible component, the restricted map is dominant.
b) No. Consider, $\operatorname{Spec}(k[x,y]/(xy))\rightarrow \operatorname{Spec}(k[x])$. You can see that the restriction to the "$x$-axis" dominates, but the restriction to the "$y$-axis" does not.