You can add a description to any field. The user can see this description in multiple places.
In the Field Picker, Looker displays this description if a user hovers over the help link, which is to the right of the field name. For example, this LookML:
Will give this result in the Looker UI:
dimension_group, Looker displays the description only once for the entire group, and not for each timeframe dimension within the group.
In addition, the description is displayed when the user hovers over the column name in a table or table chart visualization in an explore, dashboard, or Look: