Shabad Hazare is the Bani of longing for the beloved Guru. It was written by Guru Arjan Dev when he was separated from Guru Ram Das, his father for a duration of time. During that period of separation he sent these three letters to his beloved Guru and father expressing his longing for the Blessed Vision of the Guru.
Reciting this shabad brings union with the your beloved; it expresses in deep terms the true sense, the hurt of separation; the pain endured by the heart when the thought remains focused just on union and nothing else holds any meaning; when everything else loses any interest or meaning. It is an ultimate in the expression of love and longing for the Divine Guru.
The fourth part was added by Guru Arjan Dev at the time of the union with Guru Ram Das.
There is also a bani (composition) by the same name by the tenth master, Guru Gobind Singh; the full name of this composition is Shabad Hazare Patshahi 10. This second composition is to be found in the Dasam Granth.