Sometimes called the Pentagram, this is the physical form of the symbol that we draw in the air to call good energy, to seal the circle, and to bless. It's always drawn with one point up, or one point North, to show that we are channeling healing, helping, growing energies. The five points can stand for the five points of the human body; head, two arms, and two legs. So the human body does better standing on it's two legs than standing on it's head!
As the physical representation, it's a symbol of the earth, and so it stands for Earth on the altar. We put things on it to call helping energy into them, and to bless them. Traditionally it's made of wood, silver, or clay.