How to interpret the difference between natural level and steady state in Macroeconomic theory
Natural level of output is equivalent to full employment output. This does not mean full employment of all resources, there is always a natural level of unemployment even in full employment output level. 

Neoclassical economics: Steady state output is achieved when capital-labour ratio is stable or constant. In steady state, economy is not growing and also not in recession, all variales grow at the same rate i.e. birth rates equal death rates, capital accumulation rates equal depreciation rates, etc.