Language production: Communicating a message using words or symbols or signs from our memory (or internal communication system) and following the rules for combining them and using them in particular contexts.
Language processing: Understanding a message after it has been received by recognizing the concepts and ideas, the words or symbols or signs used, and the sequences and contexts in which these have been used.
Language use: This refers to producing language that is appropriate to the context and also to interpreting (understanding) language according to the context. Language use is sometimes referred to as 'pragmatics', and is often included as part of language production and also a part of language processing.