Quotidian is a fancy way of saying "daily" or "ordinary." Quotidian events are the everyday details of life.
When you talk about the quotidian, you're talking about the little things in life: everyday events that are normal and not that exciting. Going to the store, doing chores, working or going to school, and brushing your teeth are all quotidian. If you take a spaceship to Mars, that would be unusual and extraordinary: the opposite of quotidian.