Hard to translate directly, but it means to do an act of public welfare or service. In this context, the grandmothers are doing an act of public welfare (in other words, they are acting in the public's best interest) by knitting sweaters for those who are cold. The word salut can sometimes have a religious connotation, as it is also the word for salvation.

Bonjour, C'est faire don de son temps à une communauté.