Bloons become brittle and take extra damage while frozen.
~ BTD 6 Description
Embrittlement is the fourth upgrade of Path 1 for the Ice Monkey. It allows the tower to pop 2 layers of bloon and frozen bloons take extra damage from other attacks (+1 damage to be exact), including sharp objects (for a brief time after thawing out). It costs $2,550 on Easy, $3,000 on Medium, $3,240 on Hard, and $3,600 on Impoppable.
"Embrittlement" is an actual word used within the material science study field that describes the loss of ductility. Objects that suffer from embrittlement become easier to rupture when forced under pressure, similar to what the Ice Monkey's Embrittlement upgrade does to bloons.
This (and by extension Super Brittle) is the only upgrade that allows the ice from Ice Monkeys to be popped by sharp objects on a non-MOAB-class Bloon.