As far as I know, the visa is multiple entry for 5 years that is valid only if u enter US 1st for the applied reason. For this case, your parents must enter US for your graduation (before the graduation date). They can stay for 6 months and then MUST go back. Then they can enter whenever they want for 5 years but each stay should be 6 months. If there is violation to any of these, the visa will be canceled.