This is the 3312th most frequent French word.
urgence
Urgence: Emergency; a situation requiring immediate action.
Here, 'urgence' is a noun again, but referring generally to any kind of urgent or critical situation.
Il est important de savoir quoi faire en cas d'urgence.
It's important to know what to do in case of an emergency.
Here, 'urgence' is used as an adverbial phrase ('en urgence'), meaning 'urgently' or 'without delay', modifying the action of calling.
J'ai appelé le médecin en urgence.
I called the doctor urgently.
In this case, 'urgences' is used as a noun to refer to medical emergencies, specifically the area designed for urgent medical situations in a hospital.