Sky Glass does not need an aerial to function. Outside of electricity, the only thing it needs is an active, stable internet connection with a download speed of at least 10Mbps to stream content in High Definition (HD) and 25Mbps for 4K Ultra HD.
You do have the option to connect an aerial to the television though. This will be used as a fallback in the event your internet connection takes a turn for the worst, plunging you into an online blackout, so you aren’t entirely without entertainment.
You won’t have access to the same channels and services as you do through Sky Glass when watching through an aerial. Instead, it’s restricted to the free channels included with Freeview. You can see what these are using the Freeview Channel Checker.
It’s also north noting that you can’t watch Freeview while you have an active internet connection even if the aerial is connected the entire time — the aerial is intended to be used as a backup when the television can’t establish an internet connection.
Even though it’s not a requirement, you also have the option to connect an aerial to each Sky Glass Puck. Just like the Sky Glass TV, this will let you tune into Freeview channels if there’s an internet breakdown.