UTM parameters are tags added to a link's URL to identify the origin of a visit: utm_source, utm_medium, utm_campaign, and optionally utm_content and utm_term. They are case- and whitespace-sensitive, hence the need for a naming convention. A visit with no UTM and no referrer is classified as direct access.
Example
https://exemple.fr/offre?utm_source=newsletter&utm_medium=email&utm_campaign=printemps