The previous poster is correct that "roku" means "six" in Japanese. However, Japanese words can have different meanings depending on which character is used to write them. I did a search, but the brand name "Roku" was always written in the
Roman alphabet even in web pages which were otherwise entirely in Japanese. Without a character to go by, it's impossible to determine the exact meaning.
Certainly the most common and obvious meaning of Roku is "six." Other possible meanings are "a reward," and "a record or transcript."