A tagset is a list of part-of-speech tags (POS tags for short), i.e. labels used to indicate the part of speech and sometimes also other grammatical categories (case, tense etc.) of each token in a text corpus.
When creating user corpora, the recommended tagset is always preselected. Using a different tagset, if there are more than one for the language, is only recommended for advanced users. Tagsets cannot be normally changed for preloaded corpora.
Since Word Sketches, thesaurus, term extraction and trends make use of POS tagging, their respective settings (e.g. Word Sketch grammar, term grammar) have to be written using the tagset used by the corpus.