The definition of the RTM as per the PMBoK guide is as follows:
" Requirements traceability matrix:
The requirements traceability matrix links product requirements from their origin to the deliverables that satisfy them."
In a bit more detail :
A requirements traceability matrix will track each requirement from its first entry, through execution, all the way into the
operational transfer. When you’re managing loads of requirements, an RTM can help you track several characteristics for each requirement:
Requirement name
Requirement’s link to the business and project objectives
Project scope and WBS entry
Any relevant data, coding, cost, or schedule about a requirement
Requirement’s current status as active, cancelled, deferred, added, approved,
assigned, or completed
Testing activities for each requirement
Comments or notes about the requirement
An RTM can help you ensure that every requirement in the project has been created to specification. This will help in quality control processes and in scope validation
later in the project.
Hope that helps a bit.
Regards,
Markus