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.
Right now, the option is available for your Facebook, Instagram*, Twitter, LinkedIn and Pinterest posts.
*For Instagram, this is available for pages linked through Facebook. Pages linked directly through Instagram do not support this option.
You can read more here about the different ways to connect your Instagram accounts.
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.