“Free” software differs from “proprietary” software in that the users of this software have the following rights:
they can use it freely (that’s the least);
they can study it freely (including its source code);
they can redistribute it to someone else freely (there, immediately, it’s rarer);
they can modify it and redistribute the modified versions freely (there, it is extremely rare).
These freedoms are often referred to as the four fundamental freedoms of free software.
Most proprietary software can only be used freely (and again, not always if the files produced are subject to license). Some can be studied, because the source codes are available (so-called “Open Source” software), but this possibility is often accompanied by a non-disclosure clause and non-use of the techniques learned (these clauses are of course particularly intolerable and very dangerous because definitive once you have had access to the source code). In addition, the analysis of software is generally prohibited, except in cases expressly provided for by law (in particular, in France, the decompilation and analysis of software is authorized in the context of the search for interoperability between software , provided that this does not require the circumvention of a means of protection). Finally, the redistribution of proprietary software and their modification are almost systematically prohibited.
Note that free software is not free software or public domain software. On the contrary, they are protected by copyright, and the rights conferred on users are only protected by a license chosen by the author of the software, who therefore remains the owner of his work. Any use outside this license is therefore totally prohibited, whatever it is. The rights and obligations of users are therefore always specified by the software license, which the user must accept at least tacitly.
In particular, certain free software licenses bring an additional restriction: the prohibition to withdraw the freedoms granted to the user to those to whom the products are redistributed, even modified. The rationale for this restriction is simply to prevent someone from taking over the work that others have done and offered to others for free use. In other words, the freedom of some stops where that of others begins … The licenses which impose this restriction are qualified as “Copyleft” (it is an English word game with the term “Copyright” well known, and the word “left” which actually means “leaving” to others the freedoms that the license grants us). GNU project licenses fall into this category.
Finally, note that nothing prevents you from selling someone else’s work, and for non-Copyleft licenses, from transforming it into proprietary software. But to do this, it is generally necessary to provide added value, and allow customers the freedom to redistribute, even for free, your product. Obviously, under these conditions, you will not be able to exploit many people. You will only sell at the “fair price” for the service rendered (for example, shipping, marketing or other costs).