Moldavite is what's known as a Tektite.  Tektite's are a subcategory of the meteorite family, believed to be created when a meteorite impacts the Earth under a very specific set of conditions.  Moldavite was created when a meteorite hit the Earth with enough mass and velocity that it vaporized itself and the surrounding materials.  Those vapors were ejected out of the impact site into the lower atmosphere by the force of the explosion.  Those vapors then solidified and rained back down as the Tektite known as Moldavite in the Moldau River Valley of the Czech Republic.