Sato Subaru is a high school otaku that spends his whole day in his room until someday all his life was converted upside down only by playing one video game. Will he (she) be able to live his (her) life like that The cover is not mine here is the...
It is a rough estimate of how many pages your story would be if published as a paperback book. I believe it assumes double-spaced, twelve-point font, which is roughly 250 words per page.
Most paperback books are shipped with roughly 250-300 words per page. On ScribbleHub it uses that estimate to determine how many pages your web novel would make up to if published as a proper paperback.