It will increase/decrease the stat by 10%.
You can check here to see which stats are increased/decreased.
Thought this was relevant in one way or another, but:
If the increased stat of the nature is Attack, it's favourite flavour is Spicy.
If the increased stat of the nature is Defense, it's favourite flavour is Sour.
If the increased stat of the nature is Speed, it's favourite flavour is Sweet.
If the increased stat of the nature is Sp. Attack, it's favourite flavour is Dry.
If the increased stat of the nature is Sp. Defense, it's favourite flavour is Bitter.
Hope I helped. :)
Source: Above Link