Electrical panels and panel boards are to be fully contained without openings and gaps
A knockout in a junction box or circuit panel is a small, round piece of metal that gets, well, knocked out when wiring is installed. Someone wanted a hole on the panel to put wires in and changed their mind. or there were wires in that were taken out.
As a home inspector, I see them quite often. I call them out as hazardous, because a child could stick something into it, or sparks could shoot out if there should be a short circuit in the panel.
The fix is pretty straightforward; Measure the hole (because they come in different sizes), and subtract 3/8 of an inch. So if the hole is 7/8″, buy a 1/2″ plug. The plugs are hard to get in. Use a hammer.
The circuit breakers fill a rectangular hole. filling them is easy, if you have the panel manufacturer’s name and serial number with you when you go to the store.
Remember; Always turn off the power when working around electrical wires.