Migration paths allow for questions to be moved from Ask Ubuntu to other sites in the Stack Exchange network. The idea is that rather than simply closing questions as being "off-topic", then the questions can be moved to be the correct Q&A website.
These could allow us to keep our site on topic and at the same time not being 'mean' by simply closing questions and effectively preventing the asker from getting answers to valid (but off-topic) questions.
From the Stack Exchange blog:
If you’d like to make a case that a migration path should be unlocked between two sites — show us examples of those questions being closed as off-topic on the site.
So what migration paths would we like?