Alt(ernative) Text can be provided to give additional information about an image or graphic. Sometimes this text displays when mousing over the image. It is especially important for accessibility puproses, as it gives a screen reader a description to read in place of the image. If this text is not provided, the screen reader will simply say "image" or read the file name.
What Should I put as Alt Text?
Alt Text should be concise and clearly explain what the image is. In most situations, it is also helpful for Alt Text to give insight as to why the image is significant or why it was included. Many websites refer to this as capturing both the content and function of the image.
You do NOT need to use phrases such as "image of...." or "graphic of...."