It is a matrix of requirement against the test cases it is useful for tracing which test case is for which requirement.
There can be many test cases for a single requirement or a single test case can test many requirements
If the test suite is very large in case of a large and big project then it is very difficult to trace which test case is for which requirement.
Consider below traceability matrix rows are representing requirement ID’s and columns are representing test case ID’s. The mark represents which test case is associated with which requirement.