Although not shown with the altar, the cauldron is an important tool .
Traditionally, it's made of iron, and sits in the center South of the circle (and putting it on something heatproof is advised.) This one is brass, because the iron one didn't show up against the black background of these pages, and the candle flame didn't show up on white! Remember, we are practical!
It represents the Mother, and is (among other things) the Cauldron of Eternity. Anything put into it is transformed. So we put the things we wish to change about ourselves (or symbols to represent them) into the Cauldron, and burn them, so that they will change.
A fire burning in the Cauldron is known as the Balefire, and is considered sacred.