I'll type you out something from my textbook:
"the natural rate hypothesis states that in the long run the economy tends towards the natural rate of unemployment. This natural rate is largely independent of the level of aggregate demand stimulus provided by monetary or fiscal policy, Policy-makers may be able to push the economy beyond its natural (or potential) rate of unemployment, temporarily...resulting in a higher price level. The natural rate hypothesis implies that regardless of policy-makers concerns about unemployment, the policy that results in low inflation is generally going to be the optimal policy in the long run"
- Crompton P, Swain M, Hopkins S, McEacherns W (2002), "Macroeconomics - a contemporary introduction"; Thomson, Victoria.