「六」(roku) means "six" or "6". You use it to count items, people, or as part of larger numbers. For instance, 「六つ」(muttsu) means "six things". It's one of the basic numbers in the Japanese counting system.

「六(ろく)」は「6」を意味(いみ)します。数(かず)を数(かぞ)える時(とき)に使(つか)います。例(れい)：「六(むっ)つ」。