Tagging is a way of organising tiddlers into categories. For example, if you had tiddlers representing various individuals, you could tag them as friend, family, colleague etc to indicate these people's relationships to you.
A tag is in fact just a tiddler (or a potential tiddler), and it can have tags of its own. You can add any number of tags to the same tiddler.
By tagging your tiddlers, you can view, navigate and organise your information in numerous additional ways:
The coloured tag pills on a tiddler give you quick access to all the other tiddlers with the same tag, as well as to the tiddler that represents the tag itself.
If a tiddler is serving as a tag, then the Tagging tab in its InfoPanel will show you which tiddlers are currently tagged with it.
The More tab of the sidebar has a Tags tab that presents an overview of all your tags and lets you access all your tagged tiddlers.
You can use filters to create lists of tiddlers based on their tags. You can then display any combination of the fields of those tiddlers. For example, you could build a glossary by listing the title and text of all tiddlers tagged Glossary. Such lists can be formatted in any way you wish: e.g. bulleted, numbered or comma-separated.