A cauldron is a big pot used over an open fire. You may picture the witches in Shakespeare's Macbeth hovering over a cauldron of "Eye of newt and toe of frog, Wool of bat and tongue of dog" — yum!
The cauldron, while conjuring up images of — well, conjuring — actually came from the Latin root calidus, meaning "hot." We can see this root in other Latin-based languages, including the Spanish caliente and Italian caldo. In English, the word became cauldron — a creepy name for a very useful pot.