This is the 14521st most frequent Korean word.
물거품
"물거품" means "foam" or "bubbles," and figuratively, it refers to something that ends in vain or comes to nothing.
Here, '물거품이 되었다' metaphorically means that the dream vanished or failed.
꿈이 물거품이 되었다.
The dream dissolved into bubbles.
In this context, '물거품이 되지 않도록' means to avoid the plan failing or being nullified.
운동 계획이 물거품이 되지 않도록 열심히 했다.
I worked hard to ensure the exercise plan didn't become a bubble.
'물거품이었다' is used here to express that the efforts ultimately came to nothing.