What do you mean by metallic oxides are basic and non metallic oxides are acidic?

Metallic oxides are basic in nature because they react with dilute acids to form salt and water. They also react with water to form metal hydroxides which are alkaline in nature because these metal hydroxides release OH- ions in solution. On the other hand, if you look at non metallic oxides, they react with water to form acidic solutions which liberate H+ ions in solution. Therefore, non metallic oxides would be acidic in nature.