A var is a ballad. In Punjabi tradition before the Gurus it was a heroic form, sung to celebrate a battle or a fighter, with a driving structure of stanzas built to be performed aloud to a crowd. Twenty-two of them stand in the Guru Granth Sahib, one at the end of most of the larger rag sections, and they are the clearest instance in the volume of a folk form taken over whole.
The Sikh var keeps the structure and changes the subject. The stanzas remain, and before each of them stand one or more couplets, so that the performance alternates between the compressed statement of the couplet and the expansion of the stanza. Several of the vars were assigned a traditional tune, named in the heading by reference to an existing ballad, which means the Granth in these places specifies not only the measure but the melody, by pointing at a song its audience already knew.
That detail is easy to pass over and says a great deal. A scripture that identifies its tunes by naming popular ballads is a scripture composed for people who sing, in a culture where those ballads were common property. The Gurus did not build a separate musical world for religious use. They used what was in the air and expected the audience to arrive already knowing the tune.
One var in the volume was not composed by a Guru at all, but by two musicians of the Gurus' court, and it is the earliest account of the succession written from inside the community.