... usually indicate that the word is incomplete or interrupted. It's a way to show that the speaker or writer didn't finish the word or the thought.
Here are some examples:
* "I'm so.. tired." (The speaker is too tired to finish the sentence)
* "He was so.. angry." (The writer is leaving the reader to imagine the anger)
* "The train.. suddenly.. stopped." (The writer is emphasizing the suddenness of the event)
It's not a standard punctuation mark, but it's commonly used in informal writing and speech.