What is the meaning of WordPress’s recommended css classes and where are they applied?

These classes are generated by WordPress. For instance, if a user inserts an image in his post and says it must be aligned right, WP will generate code like this in the post:

<img class="alignright" src="https://wordpress.stackexchange.com/questions/247130/...">

Now, if your theme doesn’t define the alignright class, the image won’t be aligned right and the user won’t get what he expects.

You can simply copy these styles from the core and you’ll be fine. But you may also want to adapt things, such as the color of the border around an image caption.

The last two classes are there to help visually impaired people. So, if you’re using a regular screen, nothing will happen. But it’s helping others.

You may also want to read this answer by Chip Bennett on other classes that are generated by WordPress to give you more styling possibilities.