Alt text serves two primary purposes.
The first is for image recognition by screen readers. These software applications allow visually impaired users to hear the alt text.
The second reason alt text is commonly used is to rank better in search engine queries. Most websites will focus on their SEO for text and page content.
This means that your Instagram post can live a life beyond Instagram.
How to write good alt text?
Describe the image as specifically as possible. Alt text is, first and foremost, designed to provide text explanations of images for users who are unable to see them. if an image truly doesn't convey any meaning/value and is just there for design purposes, it should live within the CSS, not HTML.
Keep it (relatively) short. The most popular screen readers cut off alt text at around 125 characters, so it's advisable to keep it to that character count or less.
Use your keywords Alt text provides you another opportunity to include your target keyword on a page, and thus another opportunity to signal to search engines that your page is highly relevant to a particular search query. While your first priority should be describing and providing context to the image, if it makes sense to do so, include your keyword in the alt text of at least one image on the page.
Avoid keyword stuffing. Google won't dock you points for poorly written alt text, but you'll be in trouble if you use your alt text as an opportunity to stuff as many relevant keywords as you can think of into it. Focus on writing descriptive alt text that provides context to the image and if possible, includes your target keyword, and leave it at that.
Don't use images as text. This is less of an alt text-specific best practice and more of a general SEO-friendly web development tenet. Because search engines can't read text within your images, you should avoid using images in place of words. If you must do so, explain what your photo says within your alt text.