In dimensional modeling, what is a bus matrix?

A bus matrix is a logical construct, it has nothing to do with tables or data sources, at least not directly.

You put business processes (things that generate facts) on the rows and dimensions (attributes about events) on the columns, and then identify which dimensions apply to which process.

In totality it defines a logical model which can be translated to a physical dimensional model.

It helps you understand what dimensions to conform, what granularity you need to support, and how your various facts are interrelated, especially causally (e.g. this process produces these facts, which in turn influence this process which produces these facts, and so on ..)

Mostly it's a way to talk to the business in their own language about the types of analysis they want to do, introducing concepts like dimensions, facts, schemas, and measures in a way that is meaningful to how the business operates.