Let's run through a simple example.
You have 1 hard drive right? You are running out of space. You go buy another drive. You want to add it to the system but now you have to decide whether you want that drive to just handle storage of files, and if so which files? Well, how about this instead? You can add the drive as a pool. It then "magically" appears as if your original drive is now x gigabytes larger than it is since it is using two drives as a pool. Convenient no?
Or how about you want to backup that drive instead? Ok, add a drive to the system. Add it to the pool, tell it to mirror the drive instead. It now copies the data from one drive to the other and any changes mirror the other. If a corrupt file is on drive 1 (your original working copy) it checks the other to see if the backup is non-corrupt. If so, it opens that file, and copies the good data to the original drive as good data. It does this with Checksums of the files.
Another for the geekiness factor is RAIDZ. One drive or two can be a parity drive. Ever use PAR files? Yup very similar. You have files or a whole drive disappear? You can pretty much restore it from the parity drive if enough of the data still exists, etc.
Or how about you like having "versions" of your filesystem. You're about to update OS X to 10.5.2 and you're afraid it might break your system. So you create a ZFS snapshot. It now olds this "snapshot" of your filesystem and the files in it. You install the new update, it does indeed bork your computer. So you tell OS X and ZFS to use a previous snapshot, boom. You're back to 10.5.1 and it works.
Also the idea of using ZFS with time machine, is really really cool. Also, on the fly compression of your files. With no real performance hit. Another neat one is built in encryption. So you'll be able to have file vault at the filesystem level rather than the OS X application/OS level.