Rally good question! This is a term we use a lot but I personally don't think about exactly what it means. So let me see.....sin is transgression, or breaking God's law.
There are "ten commandments," but there is no one who can live without breaking at least one of them.
The ten commandments are meant to "convict" us of our sin, meaning, make us aware - make us aware that we need to be saved from the penalty for sin. "For the wages of sin is death; but the gift of God is eternal life through Jesus Christ our Lord." (Romans 6:23)