Definition: When you talk about something that, from your point of view, has no borders, no limit, you can use this magic phrase "The sky's the limit...".
Examples: The sky's the limit when it comes to my will to explore the whole globe. The sky's the limit for human imagination.
For some more examples, you can drop into the Oxford dictionary website: https://en.oxforddictionaries.com/definition/us/the_sky's_the_limit
Sometimes it's used in a context of money. "How high can the price of the stock go?" "The sky's the limit!"
If your parents take you out to a fancy restaurant to celebrate something, and you ask "what can I order?" they might say "Anything! Don't look at the right side of the menu! Order whatever you want. The sky's the limit!" ("The right side of the menu" is where the prices are listed).
There are other related phrases and idioms.
Suppose someone said "If only more people learned Esperanto, we would have world peace. Someone else might reply "You're dreaming! Your head is in the clouds!"
Someone who is always practical and sensible, who never has their head in the clouds, is "a down-to-earth person."
Someone who is thrilled and overjoyed by a wonderful piece of news might be "on Cloud Nine!" or "over the moon!"
A set of security laws intended to protect investors against imaginary or wildly exaggerated investments is called "the blue-sky laws."
Someone making overoptimistic imaginary plans is "building castles in the air."
