'hardy fish' is not a specific fish, rather it is a generic term used to describe what could also be called easy to keep fish... Hardy fish can better tolerate higher levels of Ammonia, Nitrite & Nitrate in the tank, and can also accept wide pH ranges for the water and water hardness conditions. In other words they are not fussy fish.
Hardy fish are therefore recommended for the beginner, because any start-up tanks that they find themselves in will have to go through a cycling period. 'using' hardy fish in this manner is called 'cycling with fish' & is not recommeded as it is quite cruel to the fish themselves - it will harm them & can kill them. Basically to cycle a tank you need ammonia in the water - i.e. fish pee. You're therefore making the 'hardy fish' swim about in it's own sh1t for a couple of months so that your filter can get to a stage where it can then clean the water!
Latter day thinking is the next obvious thing to do: 'Fishless cycling' - i.e. get some pure ammonia from a shop and use that instead of using a fish to generate it!
Andy