Original sin is the concept that, because of Adam and Eve's sin in the Garden of Eden, all humans are born with the weight of that sin on their backs. If a person doesn't accept Christianity within their lifetime then this sin will send them to hell.
Problems with Original Sin
For one, how is it fair for God to punish every human that will ever exist for the sins of a pair of people who lived thousands of years ago? The only ones that bear the weight of a sin are the ones that do the sin. For example, if a father assassinated a political leader it wouldn't be just to punish his great-grandson as well.
A larger problem arises with the set-up of the Garden itself. God created Adam and Eve without knowledge of the difference between good and evil. He then told them that they could eat anything in the garden except the fruit of the so-called Tree of the Knowledge of Good and Evil. He also gave humans free will because he wanted them to be able to freely choose him. Thus, when they ate from the tree he was able to punish them for disobeying him. However, the problem arises from the fact that Adam and Eve, being created without the knowledge of good and evil, wouldn't've known that it was wrong to eat from the tree until after they ate from it. They basically had the moral intelligence of infants. Furthermore, God is supposed to be omniscient, meaning that he knew before he even created them that they'd disobey him. If he knew beforehand then why'd he create the tree in the first place? In essence, the entire Garden of Eden was a set-up.