Alt text is a text explanation for an image. Should the image not load or someone use a screen reader, the text will allow the user to know what is on the page even if they cannot see the picture.
There are a couple ways to add alt text. The simplest ways to add alt text would be to add to your html
<img src="image_name.jpg" alt="my image file, and this is the alt text">
Now this is fine for html, but if you're using Springshare and are in the image utility, you are going to need to select the alt text function in the image manager.
If you want to add something in D2L, you can add a "description" to your image files
If an image is only decorative, either mark "decorative image" (if possible) or just put "image for aesthetic purposes only" in the alt text.