This is the 2315th most frequent Russian word.
массово
"Massovo" means "on a large scale" or "massively."
In this sentence, 'массово' suggests the widespread or large-scale nature of the action being described.
Книги этого автора издаются массово и пользуются большим спросом.
Books by this author are published on a mass scale and are in great demand.
The word 'массово' here is used as an adverb to signify that a large group of people was involved at the same time.
На площадь вышло массово людей, чтобы отметить праздник.
A large number of people gathered in the square to celebrate the holiday.
Here, 'массово' conveys the idea of something being done to many subjects simultaneously or as a group.