Japji Sahib is the composition that follows the Mul Mantar, thirty-eight pauris with a prologue and an epilogue, by Guru Nanak Dev Ji. It is recited every morning by observant Sikhs and it is the first thing in the volume.
It moves through a sequence: the futility of ritual purity, the nature of hukam or divine order, the limits of what can be described, the accumulated praise of those who have tried, and then the five khands, the realms of duty, knowledge, effort, grace and truth, through which understanding is said to move.
It is not narrative and it contains no biography, no law and no ritual instruction. It is an argument.
It is also the most translated Punjabi text in the world and the hardest. Its vocabulary is compressed, its grammar is early, and each of the standard English versions reads as a different work.
It is recited from memory by a very large number of people who could not paraphrase a single pauri, which is a fact about religious practice everywhere and worth saying plainly.
The Mul Mantar in front of it is the shortest statement in the tradition and the one most often quoted alone: one being, true name, creator, without fear, without enmity, timeless in form, unborn, self existent, known by grace. Everything after it is an unpacking of that line.
No raga is prescribed for it. Alone among the major compositions it is not set to music and is recited rather than sung, which is why it is the one bani a person can do walking, on a bus, or with their hands busy.
The closing shlok is by Guru Angad Dev Ji and not by Guru Nanak Dev Ji. It puts air as the teacher, water as the father and earth as the mother, and ends on the accounting of what each person did with the time. A composition about the limits of what can be said finishes with the plainest sentence in it.