Taxonomies can be used as fixed or free tagging terms. For free tagging taxonomies, the user is allowed to insert further own values if needed (for example, if a skill is missing from the predefined list) but you can also show a list of options.
Due to the many different terms possible within a vocabulary, the system allows free tagging for languages and skills on the CV.
From time to time, an admin can check these entries and move all relevant new terms into the main vocabulary by using the taxonomy manager. Find more information on how to add new taxonomy terms in this article: I am creating a new taxonomy for my new job board - how can I do that?