Reportedly, the 18th century Continental Congress that chose the first flag design did not have anything associated with the colors, except perhaps for inspiration and relation to the colors of the Great Seal, whose colors do have meaning. White is supposed to be a representation of purity and innocence, while the red signifies valor and hardiness. Blue multitasks a bit more, representing the color of the Chief, but also justice, perseverance and vigilance.
The current flag has 13 stripes representing the original 13 colonies. It also has 50 stars, representing the current 50 states, and has done so since for over 55 years, the longest stretch of any flag design. As the Union has grown, 26 flag modifications have happened since Independence. The current flag design was 48 stars for the Continental United States for a while, then jumped to 49 stars briefly and then 50 the following year when Hawaii and Alaska gained statehood back to back.
Further expansion of the stars in the flag will have to occur if more states are added. While the possibility is remote, occasional conversations happen regarding Puerto Rico, Washington D.C., the U.S. Virgin Islands, Guam and other Pacific territories. One interesting historical missed oppurtunity would have been Newfoundland joining the Union after WWII.