According to popular tradition, the nine stripes represent the nine syllables of the phrase Ελευθερία ή Θάνατος (“Freedom or Death”), the five blue stripes for the syllables Ελευθερία and the four white stripes ή Θάνατος. The nine stripes are also said to represent the letters of the word “freedom” (Greek: ελευθερία). There is also a different theory, that the nine stripes symbolize the nine Muses, the goddesses of art and civilization (nine has traditionally been one of the numbers of reference for the Greeks).
White and blue have been interpreted as symbolizing the colors of the sky and sea.
As might be expected for a nation as long-established as Greece, there is an extensive back story to the flag that is best told elsewhere.