It refers to an alternative text (alt text) for an image. If any case your image is not displayed by the browser then in place of the image the alternative text is displayed.
It is used by search engines to read the image as search engines may not reconize the image.
SO HOW CAN I ENTER ALT TEX TO AN IMAGE
It’s very easy.
When you are writing a post and you click on “Add Media” button to upload any picture from a location or select from the media library then you will get the following screen.
On the right hand side of the windows you can see place for entering the “title“, “caption” and “alt text” for the image.
You can also align the picture to left, right etc and select the size for the image i.e full, medium, small.