Negative freedom is freedom from government interference, such as the government not arresting people for their speech or for what they put inside their bodies.
Positive freedom is freedom secured through government action. For example, public education gives people the freedom to learn and to seek career opportunities that would otherwise not be open. If health care was single payer that would provide a great deal more positive freedom than what Obamacare does now, although Obamacare was a step better than what we had before.
Both negative and positive freedom are too neglected in the United States.