Alt tags are found in the properties window for an image. They are a description of what the picture is. When the picture doesn't show up, there will be a blank window with the info that you have written for the tag in place of the picture. A setting on your computer allows you to turn off all pictures and replace them with alt tags. This is important for individuals who are vision impaired and using your website. It allows them to know what the image is even though they can't see it. This could also be helpful for those that have the setting off in order for the website to load faster.