What do we mean when we say two elliptic curves are $\mathbb{F}_q$ isomorph?

Question

One text i'm reading has the following definition

So i guess $I(t)$ is the set of elliptic curves with $\#E(\mathbb{F}_q) = q + 1-t$, and $N(t)$ is the number of non-equivalent curves. But how is a $\mathbb{F}_q$-isomorphism defined?

EDIT: also, what he means by an isogeny class?