For example, Adamant raises attack, but decreases Special Attack.
A neutral nature like serious gives 10% to a stat (like speed) and decreases the same stat by 10%, which means it does nothing
Hope I helped
Technically it increases a stat by 10%, then decreases it b 10%.
In the end, the nature doesn't boost or decrease any stat.
Basically it does nothing as others had previously stated. It gives a 10% increase to the targeted stat and a 10% decrease to the stat, ultimately surving no real advantage.
Hope I helped!