Something nationwide refers to or covers the whole country. A nationwide shortage of wheat would affect everyone in every single state.
A gossipy news story about a presidential candidate might cause a nationwide scandal, if everyone in the whole country were talking about it. And if a drought seemed to be spreading nationwide, farmers from Maine to California would start to worry about that year's crops. Nationwide combines nation, from the Latin nationem, "origin" or "race of people," and wide, from its "extending through the whole of" meaning.