Think free as in free speech, not free beer.
When using the word 'free' in a sentence like "Free software is the future!", depending on the audience, I almost always worry I'm being misunderstood. An alternative way to phrase it is "Free and open-source software is the future!" but I see eyes glaze over immediately with that sentence.
I stumble over this word both in conversation and when writing, so I think the easiest thing to do is just use the word as I intend it and reference this page so people can learn more if they choose.
Free and open-source software (FOSS) means anyone is freely licensed to use, copy, study, and change the software in any way, and the source code is openly shared so that people are encouraged to voluntarily improve the design of the software.
It respects the 'four essential freedoms of free software':
You can read more about this on the Free and open-source software Wikipedia page.
Another very important, and less technical distinction is explained on the Gratis versus libre Wikipedia page.
The English adjective free is commonly used in one of two meanings: "for free" (gratis) and "with little or no restriction" (libre).