What is the rootvg in AIX?

rootvg is, as the name suggests, the volume group (vg) that contains / (root) and any other logical volumes you created during installation -- it's basically the default AIX volume group.

Volume Groups (VGs) are an AIX thing -- they're basically logical disks (comprised of one or more Physical Volumes (PVs). Logical Volumes (LVs -- "partitions") are created inside volume groups.

http://www.redbooks.ibm.com/portals/unix has an "LVM from A to Z" book that goes into lots more detail (as well as a ton of other great reference books for AIX & the Power family of systems -- all free :)