Question

What is higher oxides and lower oxides?

Answer 1

Higher oxidation state implies higher amount of partial positive charge on the concerned element.  
lower oxidation state ionic character is more so metal oxides are basic in nature

Answer 2

higher oxides means element who make bond with oxygen is more strong and stable.
e
for example
2Zn + O2 -> 2ZnO
here zinc oxide is more stable .
you can understand it by electro chemical series.