Direct copy & paste from https://pinboard.in/howto#bundles
Bundles are named collections of tags. They are most useful for people with large numbers of tags that they wish to group by category.
A bundle lets you see all the bookmarks tagged with one or more tags in the bundle.
For example, if you had tags apple, orange and banana, you might put them in a bundle called fruit.
This for filtering purposes, right? Like, creating a collection of tags and then have a timeline with all the posts that are inside that set of tags.
Yeah, basically. And at the top of that page would be the rest of the tags.
Of course, starting with an [ALL] then however the actor wants to tag everything.