Reza Rad provides an overview:
Microsoft Fabric introduced a new concept called Domains. Domains are more than just a separation of Fabric data items. They come with a whole lot of security, administration, and governance features, which brings the concept of data mesh into the world of data analytics using Microsoft Fabric. Domains are logical categorizations inside the OneLake. In this article and video, I will explain what domains are in Microsoft Fabric, why they are important, and their associated features and configurations.
Click through for both a video on the topic and a lengthy article.