As you would imagine if you consider this visually it looks like a bowtie - a central core connecting 2 large wedges, or groups, of equal size.
One of the groups has links that connect directly to the core, but these link are not connected back form the core. This is known as the "in" or "origination" group.
The other group is the opposite; links that are connected to by the core but are not connected back to it. This is the "out" or "termination" group.
The core itself is strongly connected, and there is a fourth group that is made up of disconnected pages without links to and from it.