Explain defence of person and property.

I assume that you are asking about the legal doctrine of
self-defense.  Under this doctrine, a person is allowed to use force (even up to force that ends
up killing someone else) in defending themselves or other people around them (or their
property).  A person who harms or kills someone in reasonable self-defense cannot be charged with
a crime.

There are limits on this right, however.  Basically, the
person acting in self-defense must generally have a reasonable cause to fear that they (or the
people they are defending) are going to be seriously harmed by another person.  In other words,
you cannot shoot someone because they look like the kind of person who might murder you. 
Similarly, you cannot shoot someone who you see picking your cell phone up off your front porch
and walking away.  But you can shoot them if they are coming towards you holding a knife in a
threatening way.

Please be aware that different jurisdictions have
different laws on this and that this explanation is simply a general discussion of the
idea.