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 Working With Regions.
This page covers some of the things that can be done with playlists as objects in their own right.
Tracks are not Playlists
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. The playlist associated with a track can be changed at (almost) any time, and tracks can even share playlists.
Some other DAWs use the term "virtual track" to define 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"), which might be confusing.
Playlists are Cheap
One thing to bear in mind is that playlists are cheap. They do not cost anything in terms of CPU consumption, and they have very minimal efforts on memory use. So generating new playlists whenever needed is recommended. They are not equivalent to tracks, which require extra CPU time and significant memory space, or audio files, which use disk space, or plugins that require extra CPU time. If a playlist is not in use, it occupies a small amount of memory, and nothing more.