Here are some defs of RTM
RTM(Requirements Trace Matrix ) is used as a test planning tool to help determine how many tests are required, what types of tests are required, whether tests can be automated or manual, and if any existing tests can be re-used. Using the RTM in this way helps ensure that the resulting tests are most effective.
Requirements tracing is the process of documenting the links between the user requirements for the system you're building and the work products developed to implement and verify those requirements. These work products include Software requirements, design specifications, Software code, test plans and other artifacts of the systems development process. Requirements tracing helps the project team to understand which parts of the design and code implement the user's requirements, and which tests are necessary to verify that the user's requirements have been implemented correctly.
The Requirements Traceability Matrix (RTM) captures the complete user and system requirements for the system, or a portion of the system. The RTM captures all requirements and their traceability in a single document, and is a mandatory deliverable at the conclusion of the lifecycle.
The RTM is used to record the relationship of the requirements to the design, development, testing and release of the software as the requirements are allocated to a specific release of the software. Changes to the requirements are also recorded and tracked in the RTM. The RTM is maintained throughout the lifecycle of the release, and is reviewed and baselined at the end of the release.
It is very useful document to track Time, Change Management and Risk Management in the Software Development.
Here I am providing the sample template of Requirement Traceability Matrix, which gives detailed idea of the importance of RTM in SDLC.