Sacristans are men and women who derive their title from the room where they do most of their work, the sacristy. This is the room at the back of the church behind the sanctuary.
They do, as it were, all the "backstage" work that is necessary for mass to go well. They prepare the sanctuary (where the altar is) for the celebration of mass, they set up for funerals and such, and they make sure everything is put away properly after mass.
Often unnoticed when they do their job well, you definitely miss them when they are not there!