stroke is how far the piston moves from top to bottom based off of the height of the piston, the length of the rods, and the shape of the crankshaft. the longer the stroke, the more displacement. it's an alternative to boring out the motor ("bore it out x-over", ie. 40-over) so that can displacement can be increased without milling the block.
if you notice in the text i quoted earlier the stock stroke of the motor is 81.4mm while the kit bumps it up to 87mm and they are working on a 90mm.
i'm not a mechanic, so this is my understanding. don't be too critical
