The Nature of Freedom
We all have a sense of what it is like to make decisions and have a will. To get more clear about what freedom is, let's think about different ways we might not have it.
One common way for people to have their freedom limited is politically. When I was growing up in the United States, a common retort to any criticism of one's behavior was "it's a free country." Any country or society puts some limitations on its citizens. We don't think we are less free because we would be punished if we were caught robbing a bank. But we do feel that our freedom is impinged if we cannot criticize the government, publish our opinions, or associate with like-minded people, etc.
Political freedom is not directly related to the question of whether humans have free will. This wall prevents me from walking in this direction; this repressive state prevents me from publishing that article; neither take away my ability to make decisions in the situation in which I find myself. Even if I am tied up, I still have free will (if I ever have free will). Don't get me wrong, this doesn't mean that constraining someone without justification isn't wrong, it's just that it is wrong for some other reason, not because it takes away free will. (It takes away our freedom to act, not our freedom to decide.)
The integrity of our decision-making process is threatened when others give us incentives which radically alter the balance of our reasons. In cases of coercion, an incentive structure is provided which has the potential to alter what we really want to do. We usually take incentives to be negative, but they could be positive too.
I'm making you an offer you can't refuse.
Offering a bribe to someone has the potential to be coercive just as much as a threat. This is why some people worry about the morality of actions such as paying women in developing countries to be surrogate mothers.
(An aside: we usually think of taking bribes as wrong, but maybe bribes have an unfair bad rap. Suppose I were a government official in charge of some approval for zoning. I am offered a bribe of $5,000. Of course, I refuse. But what if I were offered a bribe of $10 million? A case could be made that it would be unethical not to take the bribe. After all, I could save the lives of many people facing malnutrition, lack of clean water, and poor medical care with that much money. Maybe it's not whether you take the money, it's what you do with it, that's wrong).
Coercion alters the framework of our choices, and it is very often bad, but it doesn't impinge on my decision-making ability, and so doesn't threaten my freedom in the purest sense.
What might threaten our freedom? It's controversial, but popular answers are things that interfere with a person's ability to make a choice: things like hypnosis, brainwashing, and extreme psychological manipulation. Many philosophers think that these conditions can undermine a person's freedom of will because they interrupt the person's rational ability to make a choice. Some think that deeply engrained addictions -- those which a person has no real chance to override -- also interfere with a person's freedom.