Test Plan is an entity that groups test cases.
For example, you may create test plan for smoke testing and test plan for testing several User Stories . Each test case may be included in different test plans.

The following steps should be performed to create complete test plan:

1. Add test plan
2. Assign required test cases on test plan (Actions -> Add Test Cases)
3. Add test plan run (Actions -> Add Test Plan Run) to run test cases in test plan against specific Iteration or Build