I believe they simply mean the glass has a high transmission coefficient (close to 100%) for light with energies in the IR, visible and UV wavelength ranges. 

high transmisssion coefficient means no absorption.  What is the significance of it

It's simply a property of the material. It will be significant if that property is important for the application. For instance, some glasses may absorb X-ray wavelengths but be transparent to visible light, meaning they may be suitable to be used for X-ray spectroscopy in a UHV chamber where it's important for you to be able to see through it (e.g. to manipulate a sample) whilst being protected from X-ray radiation. The context will  dictate the significance of that property. In another example, if you wanted to use a material as part of a solar cell (e.g. a protective layer), you need that material to let as much light through otherwise it will inhibit the solar cell.