sampling is recording sounds from anything to use as a sound, or make an instrumental sound to add to the track. think of Jayz using Otis Redding in Otis.....so many diiferents sounds sampled from tracks recorded by Otis Redding from back in the day even his vocals were used as samples for the song.
Im not the best person to be explaining sampling....but that would be the just of it I believe.... you can sample anything that makes noise, with the right equipment.