ALT text provides context for the images in your emails. When you place your cursor over an image, the ALT text is the text that display like this:
If someone has images disabled on their emails, the ALT text will act as context as to what is supposed to be there by design.
Article here for how to write alt text in your image HTML.
In the Drag andDrop Email Template Builder, there is a section for you to add your ALT text as seen here: