E.g, radically, labor markets in the absence of a basic income system are not really free, since employees starve and lose their homes if they do not accept the best available offer. Hence companies' competition for paid labor is not really free, and most groups have an existential threat if they do not accept the terms of the employers that are willing to employ them.
There are a million aspects to this that I'm not an expert in, but many of the "free market" principles are considered in the context of contrasting a market economy vs. a planned economy.
Free market means people can voluntarily engage in transactions without the government interfering in those transactions, such as when the government sets the price of some good.