Top end and bottom end

top end- includes the piston and rings, wrist pin and bearing, and head gasket, also cyl base gasket because youll be removing the cylinder. The service interval for piston inspection/replacement is very agressive on a 125. most riders extend it a little more. if you have lost compression you can get new rings and go from there untill your next piston change to save some $. Just follow the manual , dont forget to do the ring end gap inspection per the manual, its very important, you dont want your rings to bind or spin over the locating pin on the piston due to improper gap...they may need filed a little.wiseco recomends .004 per inch of bore on the end gap -youll need feeler gages.

bottom end- usually we are refering to the big end of the rod and the crank bearings and seals. this is a much more involved job but not impossible by any means. your manual will supply limits for axial-side to side play and most important vertical up and down play in the rod. since you have to remove the engine and split the cases most people replace bearings and seals to boot. good luck.

Top end Bottom end

How about when talking about power? Is more power in the top end referring to the low gears, first and second?

most people when they are talking about top-end power or gains in top end power they are talking about the amount of power the bike makes in any gear with the throttle significantly open and the bike pulling in the upper range of the selected gear.