What is HDMI passthrough?

Pass through means that in order to get HDMI out of the receiver, you need an HDMI input.
Or, the receiver can't covert an analog input source to HDMI.

Switching allows you to pass all your video inputs via the receiver and select through the receiver what you want to watch.

some one correct me if im wrong but pass throughand swithcnig are the same thing for example you have a ps3 and a blue ray player, you hook them up to a receiver then one cable to tv. hdmi 1.3a passes audio with video. some mid to high line receivers also convert component and s- video to hdmi so you could for instance connect a svhs vcr. some high line receivers also up scale 480 1080 i to 1080 p