Oh boy, I think I might be able to help you here.
It really depends on the type of stables, but general work/knowledge would include-
-cleaning stalls (shavings and/or straw)
-cleaning/refilling water buckets, sweeping
-bringing in and turning out horses
-feeding, both hay and grain
-measuring out portions of feed for horses
-general organizing and keeping things neat
-general wound knowledge
And then it expands from that very basic list. Some barn owners like all the horses' hooves to be picked out when they come in from turnout. I'm a working student for a trainer on the east coast of the US and when I head up there in December I'll be doing all of the above plus working horses daily under saddle, cleaning out the trailer, doing trot-ups with horses to check soundness, cleaning tack (clean everything we use that day), among other odd jobs including helping to build a few cross country jumps, fixing fences, dragging the arenas, etc.
It's hard work, there's no getting around that fact, but it's worth the sweat and sore muscles.
"Always be yourself. Unless you can be a unicorn. Then, always be a unicorn."