Hi There-

It's a slide. It can be made of metal, or glass, or porcelain, (or....?) It's used in a style of playing called "Slide Guitar", especially used a lot in rock and country styles. It's a whole other kind of left-hand technique that takes a little getting used to! The sound it creates is long sliding fluid notes or chords that flow into one another.
Hope that helps!
Lisa McCormick - Guitar Tricks Instructor