Many Covenant vessels are equipped with plasma bombardment weapons. They used these weapons to get species that fought the Covenant to be exterminated or they would give up and join the Covenant, as was the case during the Unggoy Rebellion, where they quelled the rebellion after threatening to glass the Grunt homeworld Balaho.
As the war with humanity escalated and the Covenant became aware of the size of their territory, they used their weapons far more openly and burnt entire worlds in a scare tactic intended to demoralise their military forces.
Glassing is an ability used to inflicted even more damage than the normal planetary invasions inflict on their own. It is added to a number of ships to allow them to bombard enemy worlds, such as the CCS-class battlecruiser and the DDS-class carrier.