The Hebrew word kaphar means "to cover over" such as a lid and is the word for the lid of the ark of the covenant (though many translations translate this as mercy seat for no etymological reason). This word is translated as pitch which was spread over the ark in order to make it water tight (Genesis 6:14) This same word is also translated as a atonement. The word atonement is an abstract but in order to understand the true Hebrew meaning of a word we must look to the concrete meaning. If an offense has been made the one that has been offended can act as though the offense is covered over and unseen. We express this idea through the word of forgiveness. Atonement is an outward action that covers over the error.