Essentially, if you put policy options for a given area on a spectrum ranging from freest to least free, there is a very small window of options that will be considered politically feasible. That is, there are relatively few policy options a politician will support in order to garner public support and re-election. The public option for health care can be seen as falling within this window.
To learn more about the Overton Window and how it relates to the struggle for freedom, please visit www.mackinac.org