This is the name given to several books, mostly from the OT era, that do not appear in the protestant Bible because they are not considered divinely inspired. However, these legendary, historical or theological books are included in the Roman Catholic Bible, normally placed between the OT and NT. There are many proven historical and theological errors and contradictions besides practices which are not in agreement with the Bible. These include praying for the dead, petitioning 'saints' in heaven, worshipping angels and atoning for sins by financial giving. In contrast to the Bible, these writings contain no fulfilled prophecies.
Jesus never mentioned any of these teachings, nor did any NT writer refer to them in the hundreds of times the OT was quoted in the NT.
See also: Bible, inspiration.