Alternative text (alt text) is a short description that identifies an image. Alt tags appears in three different instances.
Your alt text should be as descriptive as possible but kept relatively short. The most popular screen readers cut off alt text at around 125 characters. Alt tags provide another opportunity to utilize your keywords!
This alt text is just "okay" because it isn't very descriptive. Yes this is an image of students but what are they doing? Where are they?
Good: "Students raising hands in classroom"
This alt tag is better because it is much more descriptive. There are students who are raising their hands to answer a question in a school classroom.
You have the ability to add/update alt tags for all images you add to your website. If you add an image within the text of a content editor, there is an alternative text field in the image properties (access by double clicking on the image.)
If you add a content editor image, slide show image, banner image, etc. a field will appear to add your alt text.