Poets such as the Romantics would refer to classic Greek and Roman mythology. That would resonate with the people with whom they were communicating, people with similar interests and levels of education. Certainly such references meant little to the labourers or shop clerks of the time. The meaning of those words and phrases was not part of their lives. When the general population became more literate and better educated they wanted writings, fiction and poetry, that used their concepts and feelings, but especially their language.So we find most modern poetry does not try to hide what it might be about in foreign phrases or obscure images any more. The language is direct. Certainly, sometimes more is meant than is directly expressed. Often the language seems indirect. But the words and phrases used have one meaning common to the poet and the reader/listener and understood by both.
And that is just the obvious, the surface meaning of a poem. A good poem can carry so many different levels of meaning in several ways.
We'll look at that later.