The words "One Mind" are a direct translation from the Japanese 一心 (isshin). Like many concepts, this means different things to different people in different contexts.
To the martial artist, One Mind is unity of thought.
The martial artist puts his hand around the handle of a hammer. He grips the texture of the old wood, feeling the grain against his palm as any slight sweat he might have is channeled away ever so slightly, leaving his grip secure. He lifts the implement, feeling its balance against the strength of his hands. The weight is balanced towards the head of the hammer. To feel its "balance" is to acknowledge its imbalance.
He smells the faint whiff of tempered steel and the remnants of the preservatives in the wood. The environment is ever-present: his own breath, his own heartbeat, the feel of the air around him, the feel of the ground beneath the shoes on his feet. He tastes the air. He hears his breath, and faintly, the beating of his own heart, but more importantly, he feels these things. He knows they are there because they announce their presence to his senses.
Yet, the martial artist feels something before him that cannot be seen, that cannot be heard, that cannot be smelled, that defies the taste, that cannot be touched, but that he knows nonetheless is there: a wall. This is a wall of perception, a wall of complication, a collection of restrictions around him that obstruct his spirit.
In one instant, he smashes the wall with the hammer, shattering it into a thousand tiny pieces.
By breaking down the wall of perception, he is left with nothing but himself, forgetting even his hammer, which never existed to begin with.
This is One Mind.
One Mind is breaking down the walls that obstruct our thoughts as if a tangible thing. While intangible, these walls obstruct our minds, and therefore, our actions. The martial artist breaks down these walls to free his mind and release himself from fear, doubt, and hesitation, and thus achieve unity of thought.
To non-martial artists, One Mind means freeing the self from worrying about the self. Resentments about one's position in life, the behavior of other people, and simple random luck, simply wither away, dissolving as if morning mist burned away by the rays of the rising sun. The aromatic scent of the breeze through the evergreen forest replaces the mist, leaving the mind uncluttered and devoid of complications.
The real problem is how to tear down that wall.