The bottom line is the last line of an audit, showing profit or loss. It's also the main point or idea of anything.
Originally, the bottom line of an audit said whether you were making money, losing money, or breaking even. It was the most important part. From there, this became a figure of speech for the most important part of anything. If someone is talking a long time explaining something, you might ask "What's the bottom line?" That's asking them to get to the point. The bottom line of a speech is the main argument or central idea. Bottom lines are important.