What molecule is catalase breaking down in this reaction?

$2 {H}_{2} {O}_{2} - c a t a l a s e \to 2 {H}_{2} O + {O}_{2}$

The catalase enzyme is breaking down hydrogen peroxide (${H}_{2} {O}_{2}$)

Explanation:

In a chemical equation the arrow will point towards the products (things made) of the reaction. This reaction makes water (${H}_{2} O$) and oxygen (${O}_{2}$).

The other side of the arrow is where you find the reactant (starting material). In this reaction the starting material is hydrogen peroxide(${H}_{2} {O}_{2}$).

Here is a video which shows this reaction.

Hope this helps!