Taking CBD under the tongue, or sublingually, is done because it is  the most efficient way to take CBD orally By holding CBD oil under your tongue, the CBD within will be more quickly absorbed into the bloodstream.
This concept is known as bioavailability and simply means the percent of a compound that is absorbed into the bloodstream when ingested. Taking CBD sublingually ensures that you are getting the best bioavailability and thus more accurate CBD doses.