For a group $G$ and a $G$-set $M$, I am not entirely sure what the notation $M^G$ means.
For context: Over a field $k$, if $\mathcal{F}$ is the sheaf of abelian groups over $X=(\operatorname{Spec}k)_{\acute{e}tale}$ that corresponds to the to the $G$-set $M$, then $H^0_{\acute{e}tale}(X,\mathcal{F})=M^G$ (Stack 03QU). Here, $G=Gal(k^{sep}/k)$.
Could someone please help tell me the exact definition of this notation?
Edit: The question has been answered in the comments