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