Pass-thru means that no conversion to HDMI is done for lower level sources. If you look at the VSX-81TXV you'll notice that it upconverts to HDMI, component, or S-Video, the VSX-80TXV only upconverts to component or S-Video. This means that the VSX-80TXV will pass HDMI signal through if it receives it in HDMI form, but will not convert component, composite, S-video, etc. signals into HDMI format, thus requiring your receiver to be connected to your TV through at least HDMI as well as component instead of just HDMI.
The VSX-81TXV handles the conversion to HDMI and thus can convert the other signal types to HDMI and only requires a connection of HDMI between the TV and receiver.
