Well, the simplest answer to this question is that the two states of innocence and experience are captured in the two groups of poems, and the difference between these two states is emphasised by the relationship between the parallel poems. Innocence seems to be a condition that is very similar to being a child. As poems like "The Lamb" and "The Chimney Sweeper" illustrate, this childlike state is one in which the natural and human world is regarded without fear and where we are secure and have a home. We can also equate the state of innocence in Blake's work to that of being happy with our sexuality and with our bodies. The state of innocence in this way deviates significantly from any biblical parallels with the Garden of Eden.
The collection of poems, however, point towards the way that innocence as a state is not eternal, although we can do our best to cling on to it through beauty, poetry and love. As we move into adulthood with its accompanying responsibilities, duties and cares, the state of experience supersedes the state of innocence. However, Blake appears to make it clear that this progression is inexorable, we as humans exacerbate this transition through all kinds of harshness and lack of forgiveness politically, religiously and personally. Hence the famous phrase of the "mind-forg'd manacles" that dominates "London." Blake thus points towards the way that we enchain ourselves through our customs and traditions and through our own personal outlook on life. Through this phrase Blake points towards the forces of violence at work in society at large, but he also indicates that this is a process of internalisation as we accept these forces without question and as facts of life.