Concept:Free software
Free software is software which allows its user to inspect its source code, modify it, repurpose it and redistribute it. Free software also makes no attempt to dictate how the software is to be used, thus the user may use it as they see fit, even in commercial environment. Some free software also contains share-alike clauses, which require any additions to the software to also be licensed under the same or compatible license, which is commonly referred to as w:copyleft.
Ability to inspect the software's source code is a major boon for privacy as an independent expert can independently verify that the software does what it says on the tin. You no longer have to take developer's claims on blind faith; you can take meaningful steps to verify developer's claims regarding privacy and security of the software yourself, or you can rely on unaffiliated experts and community at large to sound the alarm if your software contains an anti-feature.
Thus, free software is a gold standard when it comes to preserving your own privacy. With virtually no exception, if privacy is important to you you should almost always opt for free softeare over proprietary software.