In Importacular, required fields are fields that must be added in order to satisfy the field settings in the Raiser's Edge under Config. These include all fields required by Blackbaud for all instances of the Raiser's Edge as well as the fields made required by your organisation.
Required fields will be marked with an icon in the required column in a mapping. In the image below, you can see that the Attribute Category is required (as indicated by the red icon in that column).
Once the requirement is satisfied, the icon will turn green.
In a situation where a requirement can be satisfied by more than one field (such as either an editable addressee or choosing an addressee ID), the field will show a yellow (or amber) icon until satisfied.
For more information about required fields and how they differ from key fields, please see this knowledgbase article.