In Christianity the Original sin refers to Adam and Eve's eating the fruit of the forbidden tree. Due to this sin they were kicked out of the Garden of Eden, the paradise God created for them. Since we are, according to the church, born sinners, we die sinners, and the only way we gain acceptance-if that is the right word- in God's eyes is by being humble and asking for forgivness.
Meaning of the story
Some people attribute their expulsion from Eden as punishment for disobeying God's commandment (Gen. 2:17) "Of the fruit of the tree of the knowledge of good and evil you shall not eat . . ." Others interpret the fruit as the symbol of something so precious that Adam and Eve would risk their lives to "eat of it."