Alt text gives you the option to add text to your images making posts more accessible to visually impaired users. This text should be a description of the image displayed, keep it brief and simple.
When creating a post in Planable, after adding an image to the post, hover over it to find the option for Alt text:
After saving, you'll find the option in the bottom left corner of the image. You can hover to view the text and click to edit or remove it if necessary.