How did European leaders such as Peter the Great and Catherine the Great of Russia, Frederick the Great of Prussia, Maria Theresa and Joseph II of Austria modernize the states and societies of their countries? How did they increase the power of their own countries? How did their efforts shape the balance of power in Europe in the 18th century?
Leaders such as those that you mention essentially made their countries more powerful by making them more modern. When they did this, they changed the balance of power in Europe, skewing it somewhat more to the east than had previously been the case.
Before these rulers took power, the countries of Eastern Europe were not particularly powerful. Russia was a backwards country in terms of its economics and politics. Prussia was a small state among many in those days before Germany was unified. These rulers took power and tried to remake their countries along Western lines. They tried to institute ideas taken from the Enlightenment that had swept Western Europe. They instituted more professional bureaucracies instead of having untrained aristocrats running their governments. They modernized their armies, making them more organized and training them better. By doing these things, they made it possible for their countries to function more efficiently. This allowed them to have more money and better weapons, thus making them more powerful.
When these countries became more powerful, the balance of power in Europe changed. Before, these eastern countries had not really been meaningful players in European power politics. After these leaders, the countries of the east became just as important as countries like France or Great Britain. This was a major change in the European balance of power.