Theres are 5 groups of stats
attack sp attack defence sp defence and speed
each stat has its ups and downs :
lets say I got a charizard with a modest nature modest rises sp attack but lowers attack
but there are some neutral natures like hardy docile and bashfull
so there are 2 groups for these moves (not including status changes and stat raisin or lowering)
theres physical and special :
physical is mostly all about the attack and special is all about sp attack
flamethrower is special but flare blitz is physical
so modest will help flamethrower but no use for flare blitz
heres this sites page for natures http://pokemondb.net/mechanics/natures