Taxonomy is the science of identifying and naming species, and arranging them into a classification. The field of taxonomy, sometimes referred to as "biological taxonomy", revolves around the description and use of taxonomic units, known as taxa (singular taxon). A resulting taxonomy is a particular classification ("the taxonomy of ..."), arranged in a hierarchical structure or classification scheme.
Almost anything—animate objects, inanimate objects, places, concepts, events, properties, and relationships—may be classified according to some taxonomic scheme.
Questions having the taxonomy tag relate to the use of one or more taxonomies in the context of user experience. In the scope of this Q&A site, this often refers to the naming conventions and schemes used in scientific and analytical user experience and interface design context, such as, for example, design pattern names.
Note, that there also are the terminology and copywriting tags, which focus more on how elements of an particular artefact can be referenced, phrased or named in the context of user experience. I.e. questions about labels, headings, or the like.