This example is a sentence fragment, not a complete sentence. In particular it is a noun phrase, headed by the noun "wagons" modified by the adjective "covered" and the participle "rolling across the prairies".
Note that the word is "across" not "access".
This is a sentence fragment, and not a complete sentence. You could use this sentence fragment in a full sentence, for example:
I saw many covered wagons rolling across the prairies.
A dictionary might give an example of the use of a word that is not a complete sentence:
a cute cat with a loud purr
The example of use isn't a complete sentence.