The word is taken from the Greek language and means something that is hidden. As it relates to the Bible it is in reference to certain books of the Bible that are questioned by some Christians. The most common example of this are the books of the Catholic church that the Protestant church rejects.
The reason some books are rejected has to do with many factors. The canon of scripture left these books out however. Part of the apocrypha is the pseudepigrapha (which means false writings). These writings are rejected as they claim to be by a person who didn’t write it such as the Gospel of Thomas which is believed to have been written several centuries after the apostle Thomas.