Natures
Modest raises Sp. Atk by 10% and lowers Attack by 10%. It is applied at the very end of the formula, after IVs and EVs, so the higher the stat, the more the nature gives and takes.
The standard nature for special attackers. The lower Attack only matters when the set relies on Focus Punch or on returning damage.
Flavor
Likes Dry, dislikes Spicy. This matters for Poffins, Pokéblocks and Berries: a Pokémon eats a disliked flavor reluctantly, and healing works worse.