When exactly priority and severity come into picture in SDLC phase?

Once we start creating the test cases with the available requirements,
these test cases are executed over the final developed product, while executing  these test cases, some of them will pass and some of them will fail. For failed  test cases we need to raise a bug and while raising a bug there are few  parameters that are defined like summary of bug, environment on which this  bug has occurred, version as well as severity and the priority of the bug as  well. So severity and priority comes into picture when a bug is being raised  against a failed test case.