When you are speaking in general (with plural or uncountable nouns).
Eg: Lions are more dangerous than dogs.
Sugar is sweet.
With some phrases like at home/work, go home/to bed,
With institutions ( school, church,prison, etc.) we don’t use an article when you are thinking about the institution and the normal purpose it is used it for. If you are just thinking about the building, use a or the .
Eg: She is in prison (she is a prisoner)
They must rebuild the prison as it is too old. (the building)
Before meals, days and months .
Eg: I always have lunc h at 2 p.m.
But we say a meal. Eg: We had a meal in a restaurant.