It is believed that this composition was written by Shri Guru Nanak at his advanced age.
"Japji Sahib" is the opening or the first composition of Shri Guru Granth Sahib. This is also called as the Mool-mantra or the core principle of Sikhism.
Following verses is from the first paragraph of "Japji Sahib"
"ek om kaar"
Literary means, there is but one God. This very clearly indicate the Sikhism is a Monotheist religion.
"sat naam"
HIS name is true. Name of God is pure and clean
"karta purakh"
He is the sole creator of this world and nothing exist beyond Him.
"nirbhau"
He is without any fear
"nirvair"
He has no enemy, He has no hatred
"akaal moorat"
He is timeless and form less.
"ajooni"
He is never born and never die.
"saheband"
He is self existence
"gurprasad"
He can be understood through Guru's grace
"Jap"
Pause and meditate on Him