There are so many different definitions of freedom that there can't be a nice, prepackaged pill to swallow that will help you understand this notion. I, personally, vascillate between existentialist "absolute freedom" and poststructuralist "contingency". Allow me to explain.
Sartre's notion is that each of us are utterly free, so much so that he actually suggests that we are condemned to freedom. (Hidden in this is, of course, the notion that humans don't actually want to be free, that the correspionding responsibility is wholly overwhelming.)
The problem here is that, quite simply, there is always a context. Metaphorically, the tree grows from soil, right? Many poststructuralist thinkers (Foucault, Lacan, etc.) would suggest that our identities are wholly contigent upon our environments, that our context "calls us into being". In this sense, then, freedom is a notion that comes after our identities, and is thus severely limited.
Obviously, these two perspectives are wholly in conflict, which is why I vascillate. I would suggest, ultimately, that absolute freedom exists independently of identity, that one must lose oneself before becoming free. Thus, freedom is not only something we cannot possess, but also something we cannot understand.
Okay, I'm done. Have a cookie, it always makes me feel better.