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What does this quote mean?

"If you have built castles in the air, your work need not be lost; that is where they should be. Now put the foundations under them."

It's speaking to people who may have what others consider to be unrealistic expectations or hopes.
It's just advising that if you know what you want, figure out how to make it happen.

You have big dreams and plans now do the work needed to get you there.

Don't just plan. Also grow/build and do stuff.