There's very little that's easy n' accessible about Dante's style. You might want to read a terse Hemingway short story after you finish Inferno as a kind of palate cleanser... kind of like drinking a glass of cold milk after you finish a slice of triple-chocolate-raspberry cake.
But, much like a slice of triple-chocolate-raspberry cake, Dante's language is pretty dang delicious.
When we say it's "formal," we mean that Dante adheres to a very rigid literary form. In this case, epic conventions include tons of invocations to the Muses, epithets, apostrophes, epic similes, divine creatures, and a character list longer than all seasons of Game of Thrones put together.
The "elevated" part indicates that the text is difficult. Sentences tend to be extremely long and chock full of prepositional phases. This type of language tends to describe some larger-than-life topic—say, man’s eternal damnation—and to address it in a very serious, occasionally stuffy way.
Just take a look at this gem:
[Virgil]: "Wedged in the slime, they say: "We had been sullen in the sweet air that’s gladdened by the sun; we bore the mist of sluggishness in us: now we are bitter in the blackened mud." This hymn they have to gurgle in their gullets, because they cannot speak it in full words." (Inf. VII, 121-126)
Yikes, right? But take it slowly, line by brilliant line, and you'll realize how crazy-beautiful even descriptions of sinners "wedged in slime" can be.