Caldron is the American spelling of cauldron: a large, deep pot, big enough to cook over an open fire. In stories, a witch stirs a caldron. In real old kitchens, a household used a pot like this to feed many people at once. British English writes cauldron; American English allows caldron, though cauldron is now very common in the US as well, especially in films and books. It is an old-fashioned thing. You would not normally call a modern saucepan a caldron unless you were joking. It is the same word and the same pot — only one letter is missing.