Long story short...

Why do people say this right after they get finished telling you a long story? You can't change the past with a 10 second summary of the rest of the story, lol.

It's just a way to summarize what you said to make it easier to remember/piece together.

It just means to make it short and simple to understand what story your telling.

Cause they're just being an ass. Typically, people say that before summarizing a story. They leave all the useless or redundant information out.

TLDR where you can't skip the TL part

To make people realize that they haven't been listening to the whole story.