Q:

What's the function of the image description?

What's the function of the image description?
When we ask a question, and if we upload an image, there will be a default description place:
[![enter image description here][1]][1]
[1]: https://i.stack.imgur.com/7ykzB.jpg

I don't know the ![enter image description here] function there. Sometimes I fill it in, but there is nothing more in the question.

A:

The "enter image description here" is used to fill the alt text of the inserted image. While this text isn't shown if the image loads normally, it is useful in several cases:

For users of assistive technology (particularly screen readers) so that they can still get the gist of the image's purpose despite not being able to see the image. 
If the image fails to load for some reason or is blocked (sometimes corporate proxies block the Stack Imgur in particular), the alt text will be shown instead.

You can block images in your browser if you want to see what effect the alt text has. If you're able to provide a relatively short but accurate description of the image, you should try to do that in order to help anyone who can't see the image.
You can also see the Markdown Basics page on Daring Fireball where it shows what the image format does:
![alt text][id]

[id]: /path/to/img.jpg "Title"

Note that Stack Exchange automatically wraps the image in a link so that you can click to expand it.

A:

The text enter image description here is meant to be replaced by an appropriate value for the alt attribute of the img element that adds the image to the page. The alt attribute is read out by screen readers to blind users and by other text-to-speech software (e.g. programs used by people with dyslexia). 
Compare the markdown syntax and the HTML source code for this (randomly chosen) example.
Providing text alternatives for images and other non-text content (including areas in image maps) is a basic accessibility technique. See Technique H37: Using alt attributes on img elements for the Web Content Accessibility Guidelines (WCAG) 2.1.
It is important that the alt text provides all the information that non-visual users need to understand what the page is about. If an alt attribute is too short for this (there are no hard and fast rules for when the content of an alt attribute is "too long"; some say 150 characters), you can add a more detailed description below the image. (In HTML, there are techniques that rely on longdesc or on WAI-ARIA, but these go beyond the capabilities of markdown.)

