Tracebility matrix is a table which relates the use cases with test cases

It is used to ensure that all the requirments mentioned in the use case has been implemented and tested

For eg: The use case document for "Employee Payroll" application may contain 100 cases. In order to ensure that all the 100 cases are implemented and tested, you can write a tracebility matrix

For more info, http://en.wikipedia.org/wiki/Traceability_matrix

The 'Traceability Matrix' is used to define client requirements against work planned to ensure objectives can be meet. This is development model used within the SDLC to ensure appropriate coverage and demonstrable targets of a clients requirements, priorities and other key objectives against the software development model. Does this help at all?