Please explain the mechanism of glowing bulb.

Dear student, 
 
The filament is encased inside transparent glass to let the light shine through and also to protect it from overheating. The air inside the glass is actually a small amount of inert gas that helps prevent the filament from becoming too hot and breaking. When the light bulb has fused it means that the thin coil inside has snapped and therefore electricity cannot flow completely through the circuit.

Hope this information clears your doubt.

Regards.