A matrix is simply a type of table structure that is used to express business analysis information. It is often used when information lends itself to be categorized and when information shares a lot of similar attributes. A matrix is often the best way of displaying requirements information that needs to be sorted or prioritized. 

Let's consider a real-world example.

If you have a set of requirements that all have a certain set of elements, such as a requirements identifier, a requirements category, a description, a priority, a source, and associated business rules, you can compile and manage these requirements in a matrix format. 

Matrices may be used for data dictionaries or requirements traceability, or to support the stages of performing a gap analysis.