A sin is the act of doing something prohibited, or failing to do what is required of us. But, Scripture tells us that it's more than that... that it's our very nature.
Wherefore, as by one man sin entered into the world, and death by all sin: and so death passed upon all mean, for that all have sinned. For until the law was in the world: but sin is not imputed when there is no law. (Romans 5:12 - 13 | KJV)
It's our nature to go astray from the Lord like sheep (Isiah 53:6), and to break His laws and overstep our boundaries (Psalm 51:1, Luke 15:29). The imputation (also meaning to ascribe or attribute) of sin was charged to Adam, who all of humanity is connected to. That doesn't mean that all of us are responsible for what he did, but we share the burden and the guilt of the aftermath. Sin always brings horrendous results, and greatly affects our relationship with God, as well as other people.
Likewise, God can also impute righteousness to the account of a believer, that is the righteousness of Christ (Romans 5:21). When we finally meet Him, our record is made as perfect as Christ's.