FMF Pipes And Exhausts?

Hey, ive asked a few questions and i still dont know. im looking to increase my bottem end to mid range power on my CR125R 07. I dont know which to get so can some1 help and i have a few options...i dont know if i should get a........FMF SST pipe? or FMF Fatty? which one increases more of the bottem end power? and i also want a FMF power core 2 shorty to go with it, do u think that that silencer would go good with any of those pipes? and which is the best exhaust combo for them? (eg. would the FMF Power Core 2 shorty silencer go best with the FMF Fatty?) and i dont know what jetting is and i dont know how to do it, cause i read somewhere that u have 2 rejet ur bike if u put a new pipe on it, could someone please tell me if i bought those pipes and exhausts if i would have to ''Jet'' my bike? and could you explain what it is thankyou for your help!