Alt (short for "Alternative") text is what's read by a screen reader application for users who are visually impaired. Adding useful Alt text improves the experience for those who cannot see the images.
Alt text can be added using Object > Object Export Options (Alt Text > Custom) or via in5 > Interactive Widgets > Accessibility (for Elite & Enterprise licenses).
See the full Accessibility & SEO Techniques course on Ajar Academy for more.