Definition of know what one is about in US English:

know what one is about

phrase

informal
  • Be aware of the implications of one's actions or of a situation, and of how best to deal with them.

    • ‘He knew early on what he was about.’
    • ‘But, now you are here, may I ask if you know what you are about?’
    • ‘That's the key, you need to know what you are about.’
    • ‘In time the truth will emerge but to win big contracts you have to know what you are about.’