In music there is a type of note which is never counted — it falls between the beats, and you never count it like you would count an 8th note or a 16th note.  When I was a kid I used to skip rocks on Lake Tahoe — you’re probably done it too. Grace notes are like that rock that bounces across the surface of the water. You don’t count it, but it sure adds a lot and its fun, too.

Watch this 2-minute video and your understand easily:

http://www.playpianocatalog.com