The Second Treatise of Government remains a cornerstone of Western political philosophy. Locke's theory of government based on the sovereignty of the people has been extraordinarily influential since its publication in 1690--the concept of the modern liberal-democratic state is rooted in Locke's writings.
Locke's Second Treatise starts with a liberal premise of a community of free, equal individuals, all possessed of natural rights. Since these individuals will want to acquire goods and will come into inevitable conflict, Locke invokes a natural law of morality to govern them before they enter into society. Locke presumes people will understand that, in order to best protect themselves and their property, they must come together into some sort of body politic and agree to adhere to certain standards of behavior. Thus, they relinquish some of their natural rights to enter into a social compact.
In this civil society, the people submit natural freedoms to the common laws of the society; in return, they receive the protection of the government. By coming together, the people create an executive power to enforce the laws and punish offenders. The people entrust these laws and the executive power with authority. When, either through an abuse of power or an impermissible change, these governing bodies cease to represent the people and instead represent either themselves or some foreign power, the people may--and indeed should--rebel against their government and replace it with one that will remember its trust. This is perhaps the most pressing concern of Locke's Second Treatise, given his motivation in writing the work (justifying opposition to Charles II) and publishing it (justifying the revolution of King William)--to explain the conditions in which a people has the right to replace one government with another.
Locke links his abstract ideals to a deductive theory of unlimited personal property wholly protected from governmental invention; in fact, in some cases Locke places the sanctity of property over the sanctity of life (since one can relinquish one's life by engaging in war, but cannot relinquish one's property, to which others might have ownership rights). This joining of ideas--consensual, limited government based upon natural human rights and dignity, and unlimited personal property, based on those same rights, makes the Second Treatise a perfectly-constructed argument against absolutism and unjust governments. It appeals both to abstract moral notions and to a more grounded view of the self-interest that leads people to form societies and governments.