Here's an example of how it works:
A Windows 10 user can open a word document on their desktop, begin typing away and then realise they need to get going to work. They can simply leave their desk, open up word on their phone and then be prompted to “pick up where I left off”, allowing the user to jump directly back into working.
If the user is unaware they can do this, Cortana will helpfully inform you that you can resume working on a document or email once you’re away from your desk, making it far easier to access your workflow. This feature should work in more than just productivity apps too; For example, when using the Maps app, you can look up directions on your PC before leaving the house, and then pull up the Maps app on your phone once out and about and resume exactly where you left off on your PC.