Capital, as self-valorizing value, does not just comprise class relations, a definite social character that depends on the existence of labor as wage-labor. It is a movement, a circulatory process through different stages, which itself in turn includes three different forms of the circulatory process. Hence it can only be grasped as a movement, and not as a static thing.
Volume 1 of Capital is really about understanding what capital is from the perspective of production. As we learned, capital is not a thing, but a social relation that is mediated by things: You have somebody with money on one side, somebody with no way to live on the other, and in the end wealth is generated out of this combination. A lot of what Marx does in Vol 1. is to show how this happens.
In Volume 2 we are looking at capital as a complete cycle, in which "productive capital" -- means of production combined with human labor power -- is but one component. So we are trying to look at the whole thing, from the perspective of capital and its requirements.
And from that perspective, capital "does not just comprise class relations" -- where most of the emphasis was in Vol. 1 -- but is "a circulatory process through different stages," these being three basic forms that capital must take in order to function as capital: money capital, productive capital, and commodity capital.
For example, money capital is used to purchase productive capital, ie. money capital is money that goes toward purchasing machinery and raw materials, as well as laborers to perform the work; these elements of productive capital then create commodity capital, which is the commodity which contains within it a surplus value derived from the fact that "free laborers" are dependent on a wage to live, and so accept one less than the value of what they produce. The "value-added" commodity then goes to the market to be realized as a profit when it is exchanged for money.
So we have capital going from a money form, to a productive form, to a commodity form, and then returning to a money form as the cycle begins anew.
Though I am not far along, I suspect the purpose behind Vol. 2 is to show in later volumes that any point in this cycle can be disrupted; that meeting all of the conditions for the circulation of capital is something that requires constant vigilance on the part of the capitalist class, which in practice means heavy government intervention in the economy on their behalf.