A feature is the collectivity of tags relevant to a particular topic or interest.
The majority of features have a small number of tags, like a path with a single tag for classification (e.g. highway=footway), and perhaps a name by using name=*. But since it's a worldwide, inclusive map, there are many unusual things in OpenStreetMap and we have ways to describe nearly all of them with tags.
See Category:Features for an alphabetically sorted feature index.
See Map features for a complete single list of all features and their tags.