Parts of speech describe the categories that words are assigned to according to what grammatical or lexical function they have. These categories are examples of classroom meta-language.

Example
Noun, adjective, pronoun, verb, adverb, preposition, and conjunction are all examples of parts of speech.

In the classroom
How much metalanguage, including parts of speech, learners need to know depends on their aims. Terms such as the examples above are very useful tools when learners are developing autonomy, for example by using dictionaries more.