Unsure what part this is or how to fix/replace

Looks like the Stabilizer bar or Sway Bar link nut has worn loose.

It's the stabilizer link and if you're going to replace it your going to need 2 because you don't just do one side. There roughly twenty bucks. You're going to need a 18 mm long socket , hacksaw or cut offwheel and a strong arm. If you have air tools cut thru about half way then hit it with the air ratchet should snap off.