As others have explained, it's all about capturing scenes with extensive dynamic range: for example, a bright sky combined with very dark shadows. In the following image I had bright sky in the corner. The sun was right behind the rock on the right and the dynamic range was pretty extreme between the bright sky and the dark side of the rock. In the past, I might have tried HDR or ND grad filter, but I couldn't use HDR because of the moving surf and an ND grad filter is difficult to use with a fisheye lens. So I either had to capture this in the camera or forget about it. This is what the image looks like straight out of the camera:
The foreground is very dark, the sky almost too bright. With any other digital camera I've used, you would have serious grain problems, and loss of detail, if you tried to bring back the foreground to normal levels. Not so with the k-5. All the information is there, in the raw file; it just needs to be coaxed back to life in a raw converter like Adobe's Lightroom:
Incidentally, since getting the K-5, I've stopped doing HDRs. Really no need for it any more.