Pools are collection of posts in a specific order.
Most of the time, a pool is actually a comic (a collection of posts narrating a plot), but sometimes it can be just some pictures that the original author bundled together under the same title.
Creating a pool
Clicking on "New" in the pool page will direct you to the pool creation page.
The Name field is pretty self explanatory.
The Body field can contain anything. In general, it should display a link to the original source of the comic/collection when available (e.g. the pixiv gallery.)
Don't link to imageboards as threads over there are deleted after a while, making the link dead.
If the pooled work has a sequel or a prequel (or both), use this field to add links to it. It makes following the story easier.
The Posts field contains the ID of each post that should be added to the pool, separated by at least one space.
You need at least one post to create a pool.
Finally, read howto:pools before you create the pool.
Adding posts to pools
Posts can be added either when creating a pool, or by using the "Add to pool" command on the side of the post, when viewing it.
That command will open a window in which you can enter the name of the pool (autocomplete is supported.)
Open a pool and click on "Edit" to make changes to the pool.
Pools cannot be deleted.