Diseased enemies take X% more damage from your Icy Touch, Howling Blast and Frost Strike spells.
Glacier Rot is a death knight talent located in the first tier of the frost tree. It causes enemies who are afflicted by any disease to take up to 20% additional damage from your frost-based abilities. Since you always want to keep Frost Fever up on any target you are fighting, this generally translates into a 20% damage increase for these abilities.
Together with the Tundra Stalker talent and Glyph of Howling Blast, this talent greatly increases the damage done by Howling Blast in any extended AoE situations. The ability to have Howling Blast increase the damage of subsequent Howling Blasts by applying a disease to all targets is one of the reasons why Frost tanks excel at AoE tanking.
As both Frost Strike and Howling Blast are gained from talents deeper in the tree, this talent is virtually useless for anyone not speccing deep into the frost tree.