Help understanding syntax used in security proof
A simple way to understand it is to consider that the best algorithm we know to break CDH in $\mathbb{G}$ has advantage (id est: probability to win) is $\mathbf{Adv}^{\text{cdh}}_{\mathbb{G}}$. 

You can suppose that this parameter is very small (exponentially small).

The goal of this theorem is to prove that if an adversary could break ake of SPAKE, $\mathcal{D}$, then this advantage (probability to win) is smaller of the right expression.

By looking the right expression, you deduce it's small (because $\mathbf{Adv}^{\text{cdh}}_\mathbb{G}$ is exponentially small)