(And it's not supposed to represent any country)

My guess it's largely based on culture and philosophy....

I know that's not much to go on but you asked.....

I know that the UK's flag was made up of its constituent countries' flags, so the red + is from England, the blue is from Scotland and the red x is from Ireland (although that was never Ireland's flag, I understand.) A lot of Commonwealth countries in turn incorporate this in their own flags. 

For the US, I understand that it was derived from a British flag to which the rebels affixed white strips to identify themselves (followed by more changes later.)

And then you have the really unimaginative ones, such as pre-revolution Libya. 

A flag is a brief visual representation of a country. It should sum up a country's culture in just a few colours and shapes.

Rules for making flags:

- No more than three colours

- Basic geometric shapes

- Keep it in the standard ratio 1:2 (unless you want a square flag like the Vatican City or an entirely different shape like the Nepalese flag.) 

- Try to keep every item on the flag in an ordered fashion

Basically, a kindergartener should be able to draw your flag.

As an example of representing a nation's culture let's look at Australia's flag.

The first and obvious factor is the Union Jack in the top-left corner. This is called a Canton flag. The Union Jack ties in Australia with its colonial history with the UK. Already at first glance we know this country has British history.

Below the Union Jack is the Commonwealth Star (the big star), this highlights that Australia is apart of the Commonwealth of Nations, an intergovernmental coalition of past British territories. 

To the right is the Southern Cross, a constellation of stars only viewable in the southern hemisphere. This demonstrates Australia is located in the southern hemisphere.

The Australian flag presents its history and even location just through the use of three colours and basic shapes. Everything is neatly organised; notice how the flag has four quarters, the top-left corner is the Union Jack, the bottom-left corner is the Commonwealth Star, the bottom-right corner is the bottom two stars, and the top-right corner is the top 3 stars of the Southern Cross.

If you want to make a flag for an imaginary country, or even redesign your own flag for an existing country, use Australia's as a good example.