The Greek flag is made up a white cross on a blue background in the top left corner, with nine horizontal stripes, alternating between blue and white, making up the rest of the design. The symbolism of the cross is clear and defined as a primary symbol of the Eastern Orthodox branch of Christianity for many centuries, but the interpretations of the stripes and colors have varied over the years.
Some people believe that the nine stripes represent the nine syllables of the phrase "Eleftheria i thanatos", which translates in English as "Freedom or Death" and is actually the national motto of Greece. Others argue that the stripes were chosen to represent the nine letters of the Greek word for "Freedom", which also ties in with the national motto. Some historians, meanwhile, argue that the symbolism of the stripes represents the Muses, the goddesses of literature, art, and science in ancient Greek mythology.
As for the colors, there are also various ideas and theories to explain the selection of blue and white. Some believe that they represent the sky and the sea, with the Mediterranean Sea playing a big part in the history of Greece and the nation being well known for having one of the longest coastlines of any nation in the world.
Others argue that the choice of colors was inspired by famous figures in Greek history and mythology. Alexander the Great, for instance, used the colors on his army banners, with the legendary Achilles also being associated with blue and white. The Byzantine Empire was also said to have used the colors regularly, but there's no general consensus on exactly why these two colors were chosen for the Greek flag, and the mystery may remain unsolved forever.