What is the meaning of the red flag in Les Miserables?

A deeper explanation as to why the Red Flag became a symbol of revolution, from http://www.crwflags.com/fotw/flags/fr_revol.html#red, which got its information from M. Pastoureau in his book Les emblèmes de la France:

In July 1791, King Louis XVI and the Royal Family attempted to flee France, dressed as ordinary people. They were arrested in Varennes, on their way to Germany. It is often said that the son of the post house's owner recognized the king after a coin. Others said that the royal princesses were recognized because they were not able to walk correctly without a servant to assist them. Betrayal might be a more rational explanation.

The king was brought back to Paris. A "Republican petition" requiring the overthrowing the king was deposed on Champ-de-Mars, where the Fête de la Fédération had been celebrated on 14 July 1790.

A lot of people gathered to sign the petition. On 17 July, when the meeting turned into a riot, the Mayor of Paris, Bailly, ordered the hoisting of the red flag, which meant at that time that the mob should disperse. The National Guards shot without warning. More than 50 rioters were killed, and were immediately considered as the first martyrs of the Revolution. The red flag, "shed with the martyrs' blood" became the symbol of the Revolution by a weird inversion of its initial symbolism.

The Blue and Red parts of the French tricolor originates in the Flag of Paris.

I don't think it has anything to do with the red part of the French flag.

In general a red flag is viewed as a symbol for left wing politics and revolution

Taken from the red flag wikipedia page:

In politics, a red flag is a symbol of Socialism, or Communism, or sometimes left-wing politics in general. It has been associated with left-wing politics since the French Revolution.

It was used before the French revolution as a symbol for revolt. Taken from The story of the red flag

This (the french revolution) was not the first time that the red flag was raised, representing the revolutionary aspirations of the oppressed. The red flag has historically been raised by classes in revolt. In the struggles which rocked the rule of Rome, ultimately bringing it down, the red flag was the battle standard held high by slaves who had no way out but rebellion. And in the great peasant revolts that swept across Germany—peasant armies moved from one part of the land to another raising the red flag in their midst.

The red flag in Les Miserable is a virtual character in the production with its own story arc. It is foreshadowed in small ways as a table cloth and a cloth in a cart, a smaller flag and then a giant triumphal flag rallying the protesters, and finally as tragic reminder draped near the body on the barricade. The flag grows in size scene by scene to represent the growth of the movement and is there at the end to acknowledge the death of the protesters.