If there's one button on your camera that you could go your entire photographic career without ever pressing, it's the depth of field preview button. Nonetheless it turns out that that weird little button is rather useful. This helpful video explains its function, quirks, and best uses.
If you've ever looked at the front of your camera at a weird angle down near the lens mount, you've probably seen a little button by itself. That's the depth of field preview button, and as you can guess by it's not-so-inventive name, it well, lets you preview your depth of field. DSLRs keep the lens aperture wide open right until the moment of exposure, even if you have it set to be stopped down. This is for two reasons: it keeps the optical viewfinder as bright as possible and it allows as much light as possible to reach the AF sensor. So, if you want to actually see your depth of field, you need to ask the camera to stop the lens down to the correct aperture. It turns out there are a few quirks to this, though, so give the great video from ZY Productions above a quick watch.
Bonus tip: The DOF preview button is normally mapped to make an off-camera speedlight flash to show where the light is falling. You can also re-map it to another function should you so desire.