Hot water tank drain and sacrificial anode.
That is your sacrificial annode. You need to unscrew it and remove it. If it has corroded, replace it. They last about a year.
You also remove it to drain water tank, usually to winterize.
That’s your anode. You probably need to replace it.
I was told to make sure you release the pressure first. There should be a pressure release valve near the top, use that to drain water and pressure first, if not that rod could shoot out at you, so I’m told.
Looks like the hot water drain plug to me. Some newer units with an aluminum tank don’t actually have an anode rod so it may or may not have one. Anyway it’s probably a 15/16 or 7/8 sized plug.
Water shouldn’t be leaking from that…
I’d encourage you to look through all of the manuals (if you have them – if not, download them) and familiarize yourself with all of the bits & pieces so that you’re not stuck somewhere without an idea of what the problem could be.