This screenshot shows the sidebar in explanation for "Missing alternative text". It reads:
Missing alternative text
What It Means
Image alternative text is not present.
Why It Matters
Each image must have an alt attribute. Without alternative text, the content of an image will not be available to screen reader users or when the image is unavailable.
How to Fix It
Add an alt attribute to the image. The attribute value should accurately and succinctly present the content and function of the image. If the content of the image is conveyed in the context or surroundings of the image, or if the image does not convey content or have a function, it should be given empty/null alternative text (alt="").
The Algorithm... in English
An image does not have an alt attribute.
Standards and Guidelines