Migration means a change of place. A migration certificate is to certify that you have left a particular place(here, institution) and you are eligible to take admission into another.
Here are the different places one must require a migration certificate:
After senior secondary high school while getting admission into a university for an undergraduate course.
After senior secondary high school while getting admission into a university for an undergraduate course.
After an Undergraduate course while looking for a post-graduation course.
After post-graduation while looking for a university for PhD.