A story prompt is the seed of a story. It’s a sentence or an idea designed to spark your imagination. From that little seed, you can start to grow your idea. You could be given a name, an idea, or a sentence, but from that little idea you grow something new.
Writing prompts are a great resource, and I regularly give them on my Creative Writing For Kids videos on YouTube because of it. A writing prompt can be used to grow your story from a tiny seed into a full grown tree, as long as you know how to use them well.