It is preferable that you attend an XRD course at your institute and read appropriate textbooks before posting such a fundamental and basic question on ResearchGate. If you do not know the purpose of the experiment, why did you conduct it?
There are several informations which one can predict from XRD pattern. Those are:
1)The XRD pattern of any material can show you the crystallinity of the substance.
2) From XRD pattern one can predict the phase purity of the substance.
3) Lattice parameters, atomic positions etc. can be find out.
4) One can calculate crystallite size and lattice strain from XRD and many more.
And the maximum intense peak in the XRD pattern is a characteristic peak of a substance. It show that the atomic plane corresponding to maxima intense peak has larger multiplicity and large number of crystals etc.
You can determined crystallite size using Scherrer formula or Williamson-Hall plot rather than particle size.