Context: I am a second-year (undergraduate) physics major applying for a summer research position. The investigator is working on Quiver Gauge Theories and in response to my inquiry email he told me to

"compute the HS for the Coulomb branch of 3D $\mathcal{N}=4$ $U(1)$ gauge theory with $n$ flavors."

Assuming HS is short for Hilbert Series, I think I am approaching an understanding of what the problem is asking of me, however I cannot seem to find any concrete description of what $\mathcal{N}$ refers to. Help?