Pseudepigrapha literally means false writing.  It gets its name from the fact that the books that are classified as such have false author claims.  Such a book is the Gospel of Thomas which is not accepted to have been written by Thomas but instead written several centuries after that.  Another book is the Book of Enoch which claims to have been written by the Enoch of Genesis who "walked with God and then was no more."

While numerous accepted books of the Bible have been attacked by liberal scholars as having been written much later - for example some claim the book of Daniel could not have been written until the 1st or 2nd century BC - books that are classified as pseudepigrapha have essentially no support for their authorship claims.  

See also: apocrypha, canon of scripture