Poladroid is a Polaroid simulator. For those under 30, a Polaroid was a type of instant camera that took a photo, then ejected the photo itself--which developed right before your eyes. There's an urban legend that shaking the photo helps it develop faster, hence the line about "shake it like a Polaroid picture."
Poladroid attempts to replicate this experience exactly. When you launch it, a Polaroid graphic appears on your screen. Drag and drop an existing image onto it, and it ejects a Polaroid that develops on the screen while you watch. And yes, shaking the image with the mouse vigorously will get it to develop faster.
You can save this graphic at any stage in its development, and it will autosave once it's completed (which takes several minutes). The app's camera only holds ten photos at a time and must be restarted after that, a bit that seems to take realism too far.
Poladroid is kind of a novelty and it's difficult to see the appeal after waiting around for your first few simulated Polaroids to develop. It's most useful as a nostalgia trip, reminding you of those vacations in which waiting for that unusual photo to appear as if by magic was all that mattered. And a simulation is all you have now, since the real Polaroid company recently stopped selling the paper film.
Note: This link takes you to the vendor's site, where you can download the latest version of the software. This program is donationware. It is free to try, but the author accepts and encourages donations towards further development.