Use this phrase when you're in a discussion or debate, to introduce an idea that you think is:
- completely true
- the most important idea to keep in mind in the discussion
- impossible to argue against or disagree with
In the example above, the writer thinks that reducing your calorie intake is the only way to lose weight. Some other diets might work, but only if they cause dieters to eat fewer calories.
To really understand how "The bottom line is that..." is used, you need to see it used in a real discussion or debate. I suggest doing a search for "The bottom line is that" on the Internet to read some examples.