In the paper by Freed et al. "Topological Quantum Field Theories From Compact Lie Groups" they say

...the stack of G-bundles with connections is $\star // G = BG$...

My question is what's the notation $\star // G$? Is it the same as the symplectic quotient; i.e., take a contractible space $\star$ and quotient out by the action of $G$? Or, is this notation used with stacks (which I am not familiar).