Alternative text (alt text) are written descriptions of non-text content in a document, and are utilized by people with a visual impairment who cannot physically see images, tables, and graphs. Typically no more than a few sentences, alternative text should add to the understanding of the content in a document. When a screen reader reaches an image, table, or graph in a document, it will announce it and then read the alternative text that is provided. Without alternative text, screen readers cannot interpret the content. It is best practice to provide alternative text for all images, tables, and graphs in a document, especially for ones that are complex. Alternative text benefits all users of a document as it can provide further context and understanding for those without visual impairments as well.
Since some images are purely decorative and don’t add to the understanding of the content, you can label these images as decorative and they will not be announced by a screen reader. Software that allows you to add alternative text will have an area to designate the image as decorative. For example, there is a checkbox in Microsoft Word and PowerPoint that allows you to mark content as decorative in the "Alt Text" window:
How do I add alternative text in Microsoft 365?
