This is the temperature range at which a super cooled liquid becomes a glass. At higher temperatures the molecules are able to reorganise quickly as in a liquid. At temperatures below the transition range, the movement among the molecules virtually ceases and the resulting material is known as a glass.
Two characteristics should be noted here. The temperature range for the transition phase is dependent on the speed of cooling. The slower the cooling, the more time there is for reorganisation and so there is a lower transition temperature. The quicker the cooling of the material through the transition phase, the greater the volume of the material, i.e. it is less dense, although the more slowly cooled glass is still much less dense than the crystalline material.
Based on MIT Solid State Chemistry Notes, 7, pp.7