This text espouses what John Stuart Mill feels is most important about society and the way in which it works, which is the extent to which people are given free agency to act as they want and to not be controlled or have that freedom limited by the operations of the state and government.
In particular, what Mill offers and develops compared to other writers talking about similar themes, is that he refers to the harm principle, which states that we are free to do what we want as long as our actions do not interfere or harm those around us. Taking this principle to its logical conclusion, if somebody wanted to commit suicide, people who believed in the harm principle would say that they should be allowed to do it. However, Mill argued for a way of conceiving society that was far more integrated and recognised the way in which community means we are all intertwined in our lives. Even if one person chooses to hurt themself, that will impact those around him, negating the harm principle.
You can see from this description that freedom is a very complicated concept when we apply it to how it works itself out in a society. Mill's work therefore helps develop the argument of what it is to be free by deepening our understanding of society and community and our links with others in it.