A playlist is a list of regions ordered in time. It defines which parts of which source files should be played and when. Playlists are a fairly advanced topic, and can be safely ignored for many types of audio production, however the use of playlists allows the audio engineer more flexibility for tasks like multiple takes of a single instrument, alternate edits of a given recording, parallel effects such as reverb or compression, and other tasks.
Each audio track in Ardour is really just a mechanism for taking a playlist and generating the audio stream that it represents. As a result, editing a track really means modifying its playlist in some way. Since a playlist is a list of regions, most of the modifications involve manipulating regions: their position, length and so forth. This is covered in the chapter about regions. Here, we cover some of the things you can do with playlists as objects in their own right.
It is important to understand that a track is not a playlist. A track has a playlist. A track is a mechanism for generating the audio stream represented by the playlist and passing it through a signal processing pathway. At any point in time, a track has a single playlist associated with it. When the track is used to record, that playlist will have one or more new regions added to it. When the track is used for playback, the contents of the playlist will be heard. You can change the playlist associated with a track at (almost) any time, and even share playlists between tracks.
If you have some experience of other DAWs, then you might have come across the term "virtual track", normally defined as a track that isn't actually playing or doing anything, but can be mapped/assigned to a "real track". This concept is functionally identical to Ardour's playlists. We just like to be little more clear about what is actually happening rather than mixing old and new terminology ("virtual" and "track") into confusing terminology.
One thing you should be clear about is that playlists are cheap. They don't cost anything in terms of CPU consumption, and they have very minimal efforts on memory use. Don't be afraid of generating new playlists whenever you want to. They are not equivalent to tracks, which require extra CPU time and significant memory space, or audio files, which use disk space, or to plugins that require extra CPU time. If a playlist is not in use, it occupies a small amount of memory, and nothing more.