Final answer:
Onlookers join the 'second line' during the street procession of a jazz funeral, following behind the band and mourners, participating in the event and celebrating the life of the deceased.
Step-by-step explanation:
During the street procession from the cemetery in a jazz funeral, onlookers join the second line, which follows behind the band and mourners. The second line consists of people who are not part of the immediate family or the official funeral retinue, but who join in the procession to pay their respects and participate in the celebration of the deceased's life.
This tradition is particularly associated with New Orleans, where jazz funerals are a key cultural event. The jazz bands play somber tunes on the way to the cemetery and more celebratory, upbeat music on the return journey, symbolizing the transition from mourning to celebration.