The first sentence of the Wikipedia article for Parikh's Theorem states:
"Parikh's theorem in theoretical computer science says that if one looks only at the relative number of occurrences of terminal symbols in a context-free language, without regard to their order, then the language is indistinguishable from a regular language."
I'm having some trouble understanding this sentence. I understand that unary CFL's can be described as the union of finitely many arithmetic sequences. Does this mean that if we apply a morphism $h$ to some CFL $L$ which, say, maps $a \longrightarrow a$ and $c \longrightarrow \epsilon$ for some $a \in \Sigma$ and for all $c \in \Sigma$ with $c \neq a$, then $h(L)$ is a unary regular language? Could someone elaborate on this?