The cartoon in the link you have provided shows that the French Revolution was caused by anger among the Third Estate over their place in French society.  The cartoon shows the Third Estate carrying the other two estates on its back.  This symbolizes how the other two estates essentially lived off the work of the Third Estate and did not allow the Third Estate to have much in the way of rights.

Before the revolution, there were three estates.  The First Estate was the clergy.  It numbered about 110,000 people in France.  The Second Estate was the nobility.  There were about 130,000 people in this estate.  Finally, there were the commoners.  They were the Third Estate, numbering about 25 million people.

The Third Estate consisted of rich and poor people alike.  A person could get rich from working hard as a merchant (for example) but could never move out of the Third Estate.  The Third Estate, even though they were very numerous, had only one vote in the Estates-General, just like each of the other estates.  The Third Estate was taxed much more heavily than the other estates (this is why they are carrying the other two) but yet had very little in the way of rights.

For these reasons, the Third Estate was very unhappy with the state of affairs, helping to cause the Revolution.