Improved Counterspell is a mage talent that gives the Counterspell a silence in addition to the spell interrupt. Counterspell normally only locks a single school, and must be timed to interrupt a spell being cast. Improved Counterspell can be used at any time as a silence, stopping all casting for 4 seconds, including instant spells, without having to time it to interrupt a cast. Counterspell still retains its normal function, the silence is simply an added benefit.
Many mages take this talent for PvP utility. A 4 second silence is invaluable in stopping instant cast spells, and if the enemy is already injured, 4 seconds can be enough to get in the killing blow.
Improved Counterspell can give some confusing "Immune" messages. Some mobs, particularly in instances, are immune to silence but not immune to Counterspell. For these mobs, the "Immune" message applies to the silence component, not the normal Counterspell.