As per the official press release of Instagram:-
” We are introducing two new improvements to make it easier for people with visual impairments to use Instagram. With more than 285 million people in the world who have visual impairments, we know there are many people who could benefit from a more accessible Instagram.
First, we’re introducing automatic alternative text so you can hear descriptions of photos through your screen reader when you use Feed, Explore and Profile. This feature uses object recognition technology to generate a description of photos for screen readers so you can hear a list of items that photos may contain as you browse the app.
Next, we’re introducing custom alternative text so you can add a richer description of your photos when you upload a photo. People using screen readers will be able to hear this description. “
This video by James Ruth explains it further