Alt text is a term that is used online to describe images, usually on websites.
More recently, alt text is being used with images that are being uploaded to social media platforms. Instagram Alt text is a way to describe images when they fail to load. If you hover your cursor over the image will display the alt text if it was added when the image was uploaded.
Alt text can be added to a photo when you upload it or you can edit the details another time.
You can check out the detailed post on how to add Alt Text to Instagram images, here.