Complex sentence: The pots are traditionally made of clay (土鍋, "donabe") which can keep warm for a while after being taken off the fire or cast thick iron (鉄鍋, "tetsunabe") which evenly distributes heat and is preferable for "sukiyaki".
Split into simpler sentences: The pots are traditionally made of clay (土鍋, "donabe") or thick cast iron (鉄鍋, "tetsunabe"). Clay pots can keep warm for a while after being taken off the fire, while cast iron pots evenly distribute heat and are preferable for "sukiyaki".