@Tang Ho explains the meaning very well. I'd like to share the story behind this idiom with anyone interested in it.
This idom first appears in the chapter 尽心 of the book 孟子 (Mencius). The original texts are:
My rough translation:
Mencius says, "It is better to have no book at all than blindly believe in everything in the books (Thanks to @Tang Ho). For the chapter 武成 (a chapter of 尚书), I only believes in few pages of it. Since a man with benevolence is invincible in the world, when King Wu of Zhou, the benevolent king, was fighting against the cruel King Zhou of Shang, how could it be that there is so much blood (in the battlefield) that even the wooden club is floating (on the blood)?"
Explanations on the context:
The chapter 武成 describes that the battle of Muye (牧野之战) between King Wu of Zhou and King Zhou of Shang is so bloody that even the wooden club is floating. However, in Mencius and other Confucian's opinion, King Wu of Zhou is a man with many virtues (a paragon in Confucian's eye) while King Zhou of Shang is a brutal tyrant. The people is standing on King Wu of Zhou's side and King Zhou of Shang is doomed to be overthrown. It shall be easy for King Wu of Zhou to defeat his opponent, and the battle between the two parties shall not be so bloody.
I accept the viewpoint that we need to think over what we see and read, so I do believe that battle was bloody.