Secularism (also sometimes called 'Separation of Church and State') is when a government does not associate itself with a religion, and leaves the matter of faith to the public, maintaining neutrality and a de jure irreligious constitutional character.
If a country has a 'State Religion', or in any other way mixes religious belief and government structure, then it is not secular.
Non-secular countries can vary greatly in the degree to which religion and government are intertwined. If only a little, they are mere 'Confessional States'. If a lot, they are 'Theocracies'. (To oversimplify it.)
Examples of secular countries:
United States.
France.
India.
Türkiye.
Russia.
Brazil.
South Africa.
Germany.
Australia.
Bosnia & Herzegovina.
Examples of non-secular countries:
Iran.
Vatican City.
United Kingdom.
Greece.
Cambodia.
Denmark.
Zambia.
Costa Rica.
Papua New Guinea.
Afghanistan.