What is written in the Guru Granth Sahib?  

The writing in Guru Granth Sahib is called Gurbani: Word of the Guru. It is a collection of wisdom of the Sikh Gurus and also of many enlightened Sikhs and non- Sikhs whose philosophy conformed with the spirit of Sikh thought. It is solely the praises of God in poetical form, and the hymns are set to Indian classical musical scales. Besides praises of God, the Gurbani lays out a system upon which Sikh society is based and from which each Sikh can seek guidance. 

What does the Ik Oankar mean?   

It means that there is one infinite God, present everywhere.