From the Store description:
This is a free little app which helps printing images from apps that don't support printing natively; but have a sharing functionality.
Share 2 Print is meant to be used through sharing. Starting this app normally will show just an informational screen. Take a look at the screenshots for a walkthrough.
Of course, in practice, it's common to find that application content which can't be directly 'printed' also can't be directly 'shared', but the developer has clearly found this utility useful and it's good to see it available to the rest of us too. Maybe you can keep it tucked away for printing emergencies?
Share 2 Print gets added into the share targets in Windows 10 Mobile, then (right) whatever graphical content you want printed is fired off to the standard Windows printing system.
You can grab this in the Store here.
(via MyAppFree)