What is an 'acidic' oxide?
Oxides are acidic or basic based on their reaction with a base or an acid.
Here, in group 14 elements, $\ce{CO2}$ is acidic ie; it reacts with a base. When the oxides are amphoteric, they react with both acids and bases. 
Oxides of higher oxidation states are more acidic because of higher positive charge. On going down the group the ability to attract electron decreases and electropositive character increases. Therefore an element becomes less acidic and more basic down the group.
I did not understand your last question, but each period has a trend for their acidic and basic nature of oxides.