Domains are a very large topic. With organizations having access to more and more data to centrally manage all this data is getting unmanageable. For this reason, Data Mesh architecture is a way to decentralize business data. Fabric Domains are used to identify these business units that will be responsible for their own data. In a typical business the domains would be Sales, Marketing, Finance, IT, Customer Services etc. All the data will be on the OneLake but within this data lake the domains will provide separation of the data.
The concept of Domains does not silo the data, there is separation, but the data is still available to business users. For example, if you are part of the marketing team and you require access to the sales domain this is possible. You filter on the sales domain, and you can see all the sales data. In the diagram below the lines represent how collaboration between the domains is possible.