Here's how:
* It refers to a situation requiring immediate action: "We need to call the police in case of an emergency."
* It can be the subject of a sentence: "The emergency was handled quickly and professionally."
* It can be the object of a verb: "We declared a state of emergency."
However, "emergency" can also be used as an adjective, meaning "relating to an emergency":
* "The emergency exit is located at the back of the building."
So, it depends on the context of the sentence to determine whether "emergency" is functioning as a noun or an adjective.