Varnish is a cosmetic & protective finish for wood which to a greater or lesser extent allows the colours of the underlying wood to show through - in contrast to paint which covers it all up.
Wood filler is for filling holes and gaps in wood - most recently deployed in casa stubbs to repair an internal door lock (hey look, back on topic!) for which one of the mounting screws had lost its head. The screw was too skinny to cut a slot in its head so I had to excavate around it in order to get sufficient grip with a pair of thin-nosed pliers to be able to unscrew the thing. I then filled the resulting hole with wood filler, let it cure and then drilled a new hole for a new screw, et voila.
I think a more common use for wood filler is to fill cosmetic holes & gaps, because once cured it can be sanded and painted/varnished so that the repair ends up looking pretty much the same as the surrounding woodwork. I think that's one of the reasons why some wood fillers are available in different shades: so that it can be matched reasonably closely to the surrounding wood for cosmetic repairs that aren't going to be painted. Basically, it is a filler that once cured behaves sufficiently similarly to the surrounding wood to be dealt with using the same kinds of tools & fixings, as opposed to things like polyfilla and caulk which can behave differently when cut or sanded and so not make as clean a repair.
AFAIK wood hardener is usually intended to, er...harden i.e. make more robust wood that has become softened through getting wet and/or starting to rot. Wood with an isolated patch of water damage can often be repaired by cutting away the obviously unrepairable areas, applying hardener to stabilise the stuff around the 'wounds', filling the gaps, and restoring the cosmetic finish once the filler has cured. I treated the rotten corner of an outside doorframe that way once and it was fine for the next few years, until we replaced the whole door with a new uPVC framed one.
Wood resin I've never heard of, apart from the stuff that seeps out of pine trees.
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