I've never figured out how "migration" works, but I've repeatedly seen its existence mentioned, and I've seen migrated questions. Thus when I posted a question about the shape of the earth to math (dot) stackexchange (dot) com someone decided it belonged in history (dot) stackexchange (dot) com and somehow it got "migrated" to that site. So now I see this question posted to mathoverflow, and I wonder why it doesn't just get migrated to math (dot) stackexchange (dot) com, which seems to be where it belongs. How is that done and why would one not do it in a case like that?
You can read all about migration and how it works here: What is migration and how does it work?
As explained at that post, migrations should be done with circumspection; for a variety of reasons it's not a good idea to migrate something which is likely to be closed at the receiving site. Therefore the people best placed to judge when migration is appropriate are those familiar with the culture of the other site and who can sense whether it would be well-received there. In the case of Math.SE, my understanding is that questions that look like homework (the linked question could very well be an example) and that don't give indication that the OP put much thought or effort into it are generally not well-received there. In such cases it's better just to point to Math.SE as a better place for such questions than MO, thus giving the OP a chance to adapt the question before posting it there. Whereas in other cases, it may be obvious that migration to Math.SE (or wherever) is in order.