Meaning:

The most important factor in a situation or a decision to consider.

If you say the bottom line is xxxx, you mean xxxx is the most important or critical fact you want others to consider.

Note: The original meaning of the bottom line is the final line of an accounting report of a company or an organization.
That line shows how much money the company/organization has earned or lost during a particular period of time.

Example:

After a long heated discussion, the manager finally said, "The bottom line is, we have a limited budget and simply can't continue this project any longer."

I've heard enough of your excuses. What's the bottom line?

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