What is an 'acidic' oxide? My book says that:

Oxides in higher oxidation states of elements of group 14 are generally more acidic than those in lower oxidation states.The dioxides $\ce{CO2}$, $\ce{SiO2}$ and $\ce{GeO2}$ are acidic whereas $\ce{SnO2}$ and $\ce{PbO2}$ are amphoteric in nature. Among monoxides, $\ce{CO}$ is neutral, $\ce{GeO}$ is distinctly acidic whereas $\ce{SnO}$ and $\ce{PbO}$ are amphoteric.


First and foremost:
How can an oxide be acidic? Is it because of their reactions with other compounds? I know about Arrhenius, Lewis and Bronsted acids and bases but still can't seem to connect!

Secondly, why does the book say &quot;Oxides in higher oxidation states of elements of group 14 are generally more acidic than those in lower oxidation states&quot;?  Is there a logical reason behind this?

Is the trend mentioned in the second question followed by all groups of the periodic table and not just for group 14?
By 'all groups', I am referring to s and p block elements. I don't know much about the chemistry of the d and f block elements - I will study them in a few weeks time, but I reckon they should also follow this trend.
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.