It's a playful way of saying someone is "on top of the world."
Here are some examples of how it's used:
* "After winning the lottery, she felt like she had the world by the tail."
* "He's been working tirelessly, and now he finally has the world by the tail in his business."
* "The new CEO seems to have the world by the tail, making bold decisions and achieving great results."
The image conjures up a picture of someone firmly holding onto a powerful, potentially dangerous creature, signifying their mastery over it.