Pools are groups of posts with something in common. Unlike tags, the posts in a pool have a specific order.
The most common use of pools is for posts that are part of a series (such as a comic, or a group of images released by an artist). These pools will have purple-colored names.
Another use of pools is as collections for posts that share a common, subjective theme. These pools will have blue-colored names. Some of these pools are organized under the pool groups pages.
Creating a pool
Adding posts to a pool
To add a post to a pool, you can click on Add to pool from the post's page. A window will pop up where you can search for the pool you want by typing part of its name.
Alternatively, you can use the pool: metatag when editing a post's tags to add the post. For example, to add a post to the "Portrayed" pool, you could tag the post with pool:Portrayed.
Removing posts from a pool
To remove a post from a pool, you can go to the pool's "Edit" page, search for the post's ID in the "Posts" field, and remove the ID from the list.
Alternatively, you can use the -pool metatag when editing a post's tags to remove the post. For example, to remove a post from the "Portrayed" pool, you could tag the post with -pool:Portrayed.
Modifying a pool's order
To modify the order that posts in a pool are displayed in, click Order while viewing the pool.
For pools with over 100 posts, the Order link will not appear. To order these large pools, you should instead go to the pool's "Edit" page, search for the IDs of the posts you want to reorder in the "Posts" field, and reorder them within that text field.
source: danbooru wiki