It's a traditional antiseptic introduced in the UK in the 1920s. It has a strong, somewhat medicinal, smell. I gargled it once, and regretted it because it tastes foul.
It is effective against gingivitis and mouth ulcers and will also ward off vampires and werewolves. Actually smells better than garlic and is cheaper than silver, so win win.
It really isn't that bad to gargle but not particularly pleasant either. It is effective though and that's the real point.
http://www.capetalk.co.za/articles/303079/new-antiseptic-tcp...
https://www.wilko.com/en-uk/tcp-antiseptic-liquid-200ml/p/02...