Cassiterides is the old name for a group of islands that used to be known for their tin deposits. You may have heard of tin before, it's a metal that's used to make all sorts of things like cans for your favorite food and drink, and even electronic devices like phones and laptops.
The Cassiterides were believed to be located somewhere in the Atlantic Ocean, near the western coast of Europe. They were often talked about in stories from a long time ago, where people would go on adventures to find this special place that had lots of tin to mine.
Today, we don't really refer to these islands as the Cassiterides anymore, since we know more about their actual locations and don't need to use the old name. However, the story of the Cassiterides is still interesting because it shows how important tin was to people in the past, and how they went to great lengths to find it and use it in their everyday lives.