Shmira is a Hebrew word that means to guard. In this context, it represents guarding the body.
Helping with Shmira is a simple way to keep watch over the deceased so they are not alone. It was instituted so no one has to be alone while they are waiting between the time when they died until the time in which they are buried.
Most of the time, when people are sitting Shmira, the Shomrim (the person who is guarding) reads from the book of Psalms or read a book that may be available at the funeral home. Sometimes, it is a time of quiet reflection that may be used to pray for the family of the deceased.
This is most often the easiest aspect of caring for the deceased and their family. There is no physical contact with the deceased, and is the least intimidating area to serve in.