Interjections are words used to express strong feeling or sudden emotion. They are included in a sentence - usually at the start - to express a sentiment such as surprise, disgust, joy, excitement or enthusiasm.
An interjection is not grammatically related to any other part of the sentence.
Examples:
Hey! Get off that floor!
Oh, that is a surprise.
Good! Now we can move on.
Jeepers, that was close.