When we first interact with something we see its form. This can broadly be seen as the way in which something happens or the way in which it manifests. Behind this is the content. The content is that which manifests itself in a given form.
Take a painting of a moose. The form would be the materials of how this painting were made and how they all interact with each other. The content would be the moose.
When we see the form of something, we often instinctively try to find the hidden meaning, the content, that gives rise to this form. We should not simply get stuck at looking at the content, but we should try to find out why this content manifested in this particular form.
An example given in the text is the case of the commodity form. Finding out that the reason for the value is labor is to find the content behind the form of the commodity. The question we should ask ourselves then is: Why has labor value manifested itself in the form of commodity exchange?
The distinction between these two parts can be seen as first finding a reason for a phenomena, and then finding how would this reason gives rise to the phenomena. Take for example a lightning: First we can find out that the reason for their lightening is a difference of potential between the clouds and the ground. The second part would be to explain how this difference give rise to the lightning. Here we need to get a better understanding of electrons and their movement.
For exchange of commodities to be able to take place, there needs to be a notion of equal quality between them. The difference would only be a difference of quantity. This abstraction exists before we actually think of these things as equal, which is how the concept of money came forth. Our idea then that commodities can be compared in this abstract form is not the actual content of this abstraction, but the abstraction manifests itself in the form of a thought of value as something real.
The value abstraction taking place outside of the mind shows us that the exchange of commodities does not need the participants to think about what they are exchanging as an abstraction of what they're actually exchanging. Indeed, the value actually arise from the social context in which this exchange is happening. This social context in which value arises is only possible by actors acting as if their exchange isn't part of a societal whole, but rather as an exchange between atomic individuals ofthe things which contain an intrinsic value.
A "symptom" is when a particularity of a universality undermines that fundamental universality. An example giving is the universality of the idea of freedom. One specific aspect of this freedom is that freedom to sell your labor. This specific aspect in turn "enslaves" the proletarian, undermining the universality of freedom itself.