[Question] Purpose of marker in images?

<img width="1690" height="1346" alt="Image" src="https://github.com/user-attachments/assets/2afb853b-1eee-452a-814c-35c956956610" /> When I attach an image and edit its alt text, there's a marker showing. What's its purpose? Sorry if this is the wrong place to ask, but I haven't found any user help document.