1(lit.) as the water recedes, the rocks appear (idiom)
2(fig.) the truth comes to light
Examples
zhēn xiàng真相hěn很kuài快jiù就huì会shuǐ luò shí chū水落石出le了。
真相很快就会水落石出了。
It won't be long before we know the truth.
公司的职员设法隐瞒，但真相不久便水落石出了。
The staff at the company tried to cover up the truth, but soon it came to light.