Now for the natures:
Natures raise and lower a certain stat (except HP) by 10%. They both lower AND raise a stat 10% (Adamant raises Atk and lwers SpAtk, opposite with Modest). All Pokemon have a nature, but when deciding up to a nature for a Physical attacker with low Special attack and OK Speed, and choosing between Adamant (+Atk/-SpAtk) and Brave (+Atk/-Spe), its better to go with Adamant for that mon.