Complex sentence: The heraldic blazon for the Viscount's arms is "Azure a crescent between three mullets argent": that is, a blue shield containing a silver crescent with three silver stars around it.
Split into simpler sentences: The heraldic blazon for the Viscount's arms is "Azure a crescent between three mullets argent". This can be translated as a blue shield containing a silver crescent with three silver stars around it.