Working similar to the description tag in Team Fortress 2, it allows to the player to bestow a customized description of items.
Information can to be found in
"CSGO_Tool_Desc_Tag" "Description Tag" "CSGO_Tool_Desc_Tag_Desc" "Customizes a weapon with a custom description." "Attrib_CustomDesc" "This item has a user written description."
It is assumed that players could have been given the ability to provide a custom description to a desired weapon, similar to the Name Tag. Currently, it is unknown why it was never introduced in Global Offensive.