Secularism, also called Secularity or (adjectives: Secular or non religious) is the idea of something being not religious or not connected to a church. An example of this is the government, which is independent of any religion in many states. In some countries, such as Pakistan, Iran or Saudi Arabia, there is a state religion. In that case, the government follows the state religion. Another example is that of the United States: There, founding fathers such as Thomas Jefferson made a law that religion and government should stay separate. This means that anyone can choose to practice or not practice any religion they want, and the government cannot make them be a part of a religion if they do not want to.
The opposite of this is called Theocracy: In a theocracy the priests of religion are the rulers of the country.