That’s a pretty tough question to answer! In general, I’d say that physics is a bunch of rules, principles and models that predict how the universe and everything in it works.
This is as Dervil said a bit tough to answer, if you have any more specific questions you’d like to know about I could probably help more.
In general, physics is developed from the scientific method:
You make an observation, hmm this seems to happen if I do that
You make a hypothesis, this will always happen if you do that
You test your hypothesis, ok this does happen when I do it that way but doesn’t happen when I do it that other way way
You improve your hypothesis: This happens when you do that but in this way.
This process is done again and again and again and once you can no longer improve your hypothesis it is seen as a ‘law’ and that to the best of our knowledge it is true
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