This Japanese proverb means “mind, sword, and body are one”. This “ki ken tai ichi” is very close to the “ken tai ichi jo” of the ten chi jin ryaku no maki.
On the battlefield, the three elements must be united in order to survive the fight. Ki refers to mental energy, the soul. Ken refers to the weapon (often the sword). Tai refers to the broad definition of body. It includes not only the physical body but also the yoroi (and the horse).
When you mind is fudôshin (inmovable) and determined,
When your weapons move as if they were natural extensions of your physical body,
When your body is reliable because of hard and stenuous trainings,
Then you are ichi, one, united; and when unity is achieved you can become zero, mushin