Seva is unpaid work done for the sangat: cooking, serving, washing dishes, cleaning shoes, sweeping the parikrama, cleaning the sarovar.
It is not charity and it is not volunteering in the western sense. It is a religious obligation that anyone may perform, and the tasks with the lowest status are the ones most explicitly valued, which is the point.
At the Golden Temple the shoe-minding and the dish-washing are done by people of every background including the wealthy, and this is a deliberate inversion.
The institution scales. The langar at the farmers' protest camps, the disaster relief operations that Sikh organisations run at floods and earthquakes worldwide, and the pandemic oxygen supplies were all seva as a working method rather than as a metaphor.
It is the most exportable thing in the Sikh tradition and the one non-Sikhs most often notice first.
The langar at the Golden Temple serves on the order of a hundred thousand meals a day and more on festival days, and it is cooked, served and cleaned largely by people who arrived that morning to do it.
Kar seva, the voluntary rebuilding of a structure, has cleaned sarovars and rebuilt buildings across Punjab. It has also destroyed historic fabric, because a volunteer force with cement replaces rather than conserves.
The obligation is on everyone and it is not means tested. A man who owns a factory washes dishes beside a man who owns nothing, which is the arrangement and the argument at once.