Your Icy Touch does an additional X% damage and your Frost Fever reduces melee and ranged attack speed by an additional Y%.
Improved Icy Touch is a Death knight talent located in the 1st tier of the Frost Tree. It is a requirement for taking Icy Talons deeper in the tree. On its own, the talent is primarily useful for tanks, although it has some use in player versus player combat too by slowing down your opponent. For a tank, the talent is valuable both because it increases the damage (and thus threat) caused by Icy Touch and because it increases the slowing effect of Frost Fever, which helps reduce incoming damage. Because of this, the talent is popular even with Blood and Unholy specced tanks.