It's a file system. A file system is essentially the software that stores files on a disk, determining where to put them, and how to find them. Macintoshes normally use HFS+ for their file systems. Mac OS X also supports a file system called UFS, but almost nobody uses it. HFS+'s origins go back to later versions of classic Mac OS, where it was an upgrade to HFS, which in turn was designed back in the late 1980s, when most Macs primarily used 400k-1.4M floppy disks.
ZFS is an extremely advanced file system that supports a lot of things HFS+ doesn't. Many of them fit into the Macintosh "Just works" philosophy. For example, you can add a disk to your computer, and configure your existing "drive" to be spread over both the disk that was already in your computer, and the new one, without reformatting or losing any files. So by adding the disk, you've just added space to your computer, and it was completely transparent. No reorganizing of your disk is necessary.
ZFS is more future proof than HFS+ which means future Macs will be able to interoperate better with current Macs. It uses 128bit values to store things like file positions and sizes. What this boils down to is that you can have more bytes in a file than there are atoms in the universe, and ZFS will handle it gracefully. Of course, you'll need a disk that big, which you'll not be able to get, because, well, you'd need a lot of atoms
Older file systems generally get "upgraded" every years with a batch of hacks to support larger disk sizes than were anticipated when the file system was written. That will not be necessary with ZFS.
ZFS supports the same kinds of things you're used to in Mac OS X, such as long filenames, directories, "meta data" (icons, file type information, default application, etc), and other useful extras.
It's extremely fast, and has little or no legacy issues that would slow it down.
Does that help?