A kettle is a pot made for boiling water. Most kettles are metal, with a lid and a spout. If you're in the mood for a cup of tea, it might be time to "put the kettle on."
You can use the word kettle to mean "metal cooking pot," although it's mostly used for the kind you use to make your tea or instant soup. If you want to comment on the incredible awkwardness of a situation, you can use the colloquial phrase, "Well, this is a fine kettle of fish," and if you criticize a friend for something you're also guilty of, she might say, "That's the pot calling the kettle black!"