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Oversquare is an engine with a shorter stroke than it's bore. Bore is the diameter of the combustion chamber for each cylinder, while stroke is the depth. The cylinder moves up and down inside this combustion chamber.
If it helps, you can imagine the piston in an engine as your hand, working a bicycle pump going in and out (or up and down).
In an engine, in each engine revolution, the piston will go UP once and DOWN once. At 2000 RevolutionsPerMinute this is once every 0.03s. If you have a short stroke (say, 90mm), Moving 90mm (twice) in 0.03s won't require the piston to move too fast. If you have a larger stroke of 200mm, you have to move that larger distance (twice) still in 0.03s - so it's going to have to move much faster, which equals more momentum, friction and more stress for the longer stroke engine at 2000 rpm.
So, a shorter stroke engine can rev higher with less stress (think BMW here) The disadvantage is that the less momentum mentioned above equals less torque. Japanese high-revving banzaiboxes have short stroke, high revving engines and no torque. No variable valve timing can counter for this (as I am currently discovering in my VVT Audi V6) - it is physics. VWs tend to use long stroke engines. Low-revving and boring maybe, but they're nice and torquey.
(NB my maths is probably wrong)
FTR, it is generally acknowledged that the Subaru flat four is a naturally un-torquey unit - even with the turbos. I've only driven 1 or 2 but would probably agree.
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