Lots of free scripts and executors ask for a key before they run. You get one from the creator's key page, which shows a few ads or small tasks, then paste it into the script or executor. It's how creators earn something from free scripts.
A key usually lasts somewhere between a few hours and a day. When it runs out, you just get a new one. Tired of keys? Look for keyless scripts, or use a paid executor that skips its own key.