In the field selector, you can choose any field from the root entity. You can also expand related entities and choose one of their fields.
Let's imagine a simple data model with the following entities: Orders, Products and Customers
If you wanted to build a report Total orders by month, you would choose Orders as the root entity. This way, you can use any field from the Order entity on your report, such as the Order Date and the Total Orders.
However, if you wanted to build a report Total products by product type, you would choose Products as the root entity.
You can also use fields that belong to entities related to the root entity. For example, in the screenshot below, Orders is the root entity. Because there is a relation between Order and Customer, you can expand the Customer table and use the Customer status field on your report.