Associations are a term used to map content with accounts via a term. A common term would be "Cast". Videos normally have people in them, so you can associate the people with the videos via the "Cast" term. Other common associations are director, producer, writter, production company, etc.
You have to first create associations on the associations page. Once they are created, you can make associations with the appropriate content.
You can make associations in batch. Associations can be made in batch under the "bulk operations" widget on the "browse content" page.