Ki means many things to many people. There are many ways of defining it, ranging from scientific and bio-mechanical explanations to extremely spiritual viewpoints, and people's feelings about it run the gamut of complete disbelief to mystical adulation.
O Sensei believed in Ki, and he apparently talked and certainly wrote about it a lot. He did take a rather mystical approach to it, which can be rather hard to understand, and perhaps even harder to put to use in the actual practice of Aikido, let alone in daily life.
I'm not going to debate the reality of Ki here, I'm just going to offer one way to look at it that may help some people relate to it, and perhaps even offer a way to bring it into their realm of experience.
Let me use a metaphor to start.
Imagine a house. Generally speaking, a house has an exterior structure. In the normal sense, there is air all around the house, and there is air inside the house. Air is air - there is no difference between the air outside and the air inside. (Unless something has affected the immediate state of the air inside or out, but that will be addressed elsewhere.)
The only difference in perception between the inner air and the outer air is caused by the walls and roof of the house being there; the structure as we see it, and experience and live within it. This is what creates two localities and causes a perception of duality. However, it is very easy to see, in this kind of example, that in fact there is no real difference between the air inside and the air outside. If we understand that the structure doesn't truly divide the two spaces, we can see that the air is "one entity," one vast "body" that is moving, flowing, circulating.
So it is with Ki.
One way to look at Ki is as the fundamental energy, flow, and life force of the universe. There is Ki all around us, flowing everywhere, like air. There is Ki inside us; our sustaining energy. The body, in this case, as we perceive it, is like a house. It perceptually separates us from knowing the unity of our inner Ki with the outer Ki. This perception keeps us in a state of duality. To transcend this, all we need do is step outside of this perception of "ourselves" as being the dividing entity between inner and outer Ki. When we do this, we can know that the Ki around us and the Ki within us is the same, and that it is unified just like the air inside and around a house.
By being kinesthetically aware of the feeling of flow within us, emanating outward from the Hara, or center, and learning to trust that more than the conventional muscular processes that we usually rely on to support our sense of physical reality, we can release our total reliance on the body and open up to the experience of this vast resource that is flowing everywhere. We can draw on the Ki if we let go of our immediate attachment to our physical and personal references as the sole source and focus of our power and movement, and let ourselves experience and be aware of the fullness of this "universal" Ki flow, within and without. This is the "key" to using Ki in Aikido, or in life.