Your Corruption ticks have a 6% chance to increase your spell casting speed by X% (6, 12, 20) for 10 sec.
Eradication is a warlock talent in the Affliction tree, you will need to spend 33 points in Affliction to max it. This talent gives Corruption's damage over time ticks a 6% chance to increases casting speed by (6%, 12%, or 20%) for 10 seconds. These percentages correspond the the amount of points spent in the talent. See the Rank table below for more information.
Patch 3.1.0 (2009-04-14): Eradication: Whenever you deal damage with Corruption, you have a 6% chance to gain the Eradication effect. The Eradication effect increases your spell casting speed by 6/12/20% for 10 seconds. Internal cooldown removed.
Edit
Patch 3.1.0 (2009-04-14): Eradication: Whenever you deal damage with Corruption, you have a 6% chance to gain the Eradication effect. The Eradication effect increases your spell casting speed by 6/12/20% for 10 seconds. Internal cooldown removed. 
