What are the requirements traceability matrix?

Requirements Traceability Matrix is a mechanism to ensure that every testable requirement has at least one Test Case.

Requirements Traceability Matrix is a document (usually an excel sheet) wherein you list all the requirements and write down the associated test cases. One is basically mapping the test cases created by QA to the requirements BA wrote. Each requirements can/may have more than 1 test case. The only reason this document is prepared is to confirm that none of the requirements have been missed by QA. This document will come in handy in analyzing how a defect crept into production site - was it a design flaw, or missed out by QA's test case, missed by dev, or a masked bug etc.

For example, I have a requirement: Click 'submit' button to submit user's information.
QA will write 3 test cases: one to check if clicking on submit button will display a thank you message or whatever the next step is. Second to check if the information is submitted to the database (back end testing) and third to check for error messages when user input incorrect information in the form.
So in traceability matrix, you will include the requirements ID, requirement itself, and these 3 test cases ids and the titles.

Not every project requires this document because it takes time and effort to create this document.
Let's look at the projects that may/will need the said document:
- Projects that follow waterfall, v-model etc.
- Enterprise applications (huge apps) like insurance, large eComm, banking, finance etc
- When companies want to become CMM certified (again they will project only huge projects to get this certification)
- Any critical applications that have zero tolerance for bugs/errors - defense, flight control (since we have fly-by wire in cockpits now), banking, etc. - that require hawk eyes

To mapping between requirement and the test cases.Two types of traceability forward and backward traceability