What does an exclamation mark mean at the end of a sentence or word?
Exclamation marks are used for emphasis.
Exclamation marks make sentences/phrases/words more dramatic.
It expresses more emotion. It can make someone seem more passionate, excited, upset, or angry.
Just look at these examples:
Surprise! vs. Surprise.
If someone is extremely excited (maybe they're surprising you for your birthday), they'll usually shout out surprise! instead of simply saying surprise in a normal tone.
What! vs. What?
Generally, if someone tries to get our attention or you didn't hear a question/comment, you might respond with "what." What! is often used when something outlandish is said.