Hi, here it means socially acceptable behaviour.

Since you Mr. HiSun are Chinese, suggest you to read kind of Daoism, 'natural behavior' is a big topic, which is referred to as '无为' in this transcendent old Chinese philosophy.

I think it speaks about something or someone's behavioural "instinct" - (Inborn pattern of behaviour often responsive to specific stimuli). it's like, "when you corner a cat, you'll get a scratch." panicking during an earthquake for an instance, is an example of man's natural behaviour. :)

just being yourself; not "putting on airs", not trying to deceive.