A tussock is a clump or tuft of something, like the tussock of grass at the top of a hill.
Your grandpa might have an entirely bald head except for the tussock of gray on top of it, and your yard might consist of dandelions and one tussock of tall grass. It's most common to use this noun, in fact, for grass that sprouts taller than the surrounding growth. When tussock was originally used, in the 1540s, it meant "a tuft of hair." Its origin is uncertain.