Interjections are used to communicate an extreme emotion which is difficult to verbalize, or to get someone’s attention. They’re usually found at the beginning of the sentence. Because they are not grammatically connected to the rest of the sentence, they are often followed by an exclamation mark (or sometimes a comma).
Gee, I love old movies!
Whoa! That’s a lot of homework!
Oh, drat! It seems we forgot to pay the electrical bill.
Sometimes, we use a word which usually has meaning, but in the context of the sentence it’s merely for emphasis.
What was I talking about? Oh, yes, now I remember….
Here, yes is not actually being use as an affirmation (to answer “yes” to a question). It’s just an interjection.
We thought Max wasn’t coming to the family reunion and then, good lord, there he was!
Good lord isn’t being used to address any god; it’s just adding emphasis to the fact that Max had unexpectedly arrived at the family reunion.