Pass thru simply means the output would be the same as the input. For RF it means you hook your antenna to the RF input and the same signal will be on the RF output(to run to your TV or another device).
For baseband video(composite/S-video) it would mean if you hooked something to the DVDRs IN it would be available to run to something else by hooking it up to the OUT.
I believe all DVDRs work this way, not just the ES-15(which I have 2 of). The ES-15 was a decent basic DVDR, I purchased both mine for $99 USD in '07 and haven't had a lick of problem with them, although u99 errors seem to be the most common problem.