Militarism refers to a period of time when countries begin to build up the size of their army and/or navy. Since the army and navy are part of the military, the name is an appropriate one.
Countries have several reasons for building up their military.
One reason is to impress the other countries of the world. Sometimes this is done just to send a general message that the country is a strong one.
At other times it is meant to send a message to the rivals of that country that no country should try to intimidate it. The United States sent its navy, called the Great White Fleet, around the world to send a message that were now a strong country.
Another reason why countries build up their military is because they plan to use it in war. At the end of the 1800s, Germany and Italy built up their military because they wanted to get colonies. Since there were few lands left to colonize, they realized they might have to fight other colonial powers. As a result, France and Britain also built up their military.
Militarism was a cause of World War I and World War II. Sadly, often times when a country builds up its military, it is because the country plans to use it in an aggressive manner.