The free market
A free market is an environment in which people are free to buy from and sell to each other. No-one is forced to buy, no-one is forced to sell, but if buyer and seller are both willing, no-one stops them from making a deal.
A free market exists naturally unless someone uses force to coerce people.
Governments routinely prevent a free market in particular goods or services in the belief that people will buy products that are harmful to them if they're allowed to buy freely from anyone.
The impulse to protect people from themselves is well-intentioned, but libertarians believe that people should have the freedom to choose. If they make a choice that seems strange or unwise to others, they should nevertheless be allowed to make it; they may be wrong, but they may be right.
David Friedman gives the example of timolol, a beta-blocker drug used to prevent heart attacks. This was widely used in other countries but banned as dangerous in the USA for over a decade until it was finally approved in 1981. When approved, it was estimated that it would save seven to ten thousand American lives per year; which means that banning it had caused up to a hundred thousand people to die unnecessarily.
It is dangerous to take a drug that other people regard as unsafe; but it can save your life. People should be allowed to make their own decisions about their own lives.
Property
A free market is of course based on the concept of property. You can have some kind of civilization without a free market, but I don't believe you can have a civilization of any kind without the concept of property.
A pre-agricultural society of hunters and gatherers can get along without property. They produce nothing. They eat the food they find in the wild. They live in warm enough places to get along without houses or clothes. But it takes a lot of warm, fertile land to support a small population of hunters and gatherers. If humanity reverted to that system, world population would rapidly and unpleasantly drop to a very small fraction of its current level.
Even a society of subsistence farmers can't exist without property. If the food that farmers grow for themselves is taken by others, they revert to hunting and gathering, or they starve to death. Either way, no more farming is done.
A communist state without private property could probably function after a fashion — though very badly, as we've seen in the countries that have tried it. But the concept of property still exists in such a society: it's just that all property is owned by the government.