This is the 10148th most frequent Russian word.
белорусах
"Белорусах" is the prepositional plural form of "белорус," meaning "Belarusians" (people from Belarus).
Here the word 'белорусах' refers to Belarusians in the plural form in the prepositional case used after the prepositional phrase 'о' (about), forming a part of the discussion topic.
Я читал эту статью о белорусах.
I read this article about Belarusians.
The usage of 'белорусах' here again represents Belarusians in the plural prepositional case, within the context of sharing historical accounts.
В музеях, рассказывают истории о белорусах.
In museums, they tell stories about Belarusians.
The word 'белорусах' again refers to Belarusians in the prepositional plural form, used to describe the content or subject matter of the book.