Softrooting is the process of adding an additional set of applications to the internal memory card. The primary reason for doing this is that the current applications do not meet the needs of the user. Some do this because of the interest in messing with hardware. The current library application that comes with the nook is absolute crap when it comes to the sideloaded content (non-B&N books) so for many of us, the nook would be worthless without softrooting.
If you get a nook with 1.4 firmware the process is pretty easy but the instructions are not complete and have a lot of out of date information. For the newer firmware, the process is much more complicated.
The odds of getting the 1.4 firmware are slim and only apply to the refurbished nooks sold at buy.com and others so it would be better to wait until the 1.5 firmware comes out and see if they solved the library problems. B&N knows this is an issue and they have somewhat addressed it in the color model, but it will be a few weeks before we see if they fixed things for the older model.