In centralized, closed-source programs and applications, only those granted access by the central authority can inspect the code to see exactly how it works. When using closed-source applications, the user is forced to trust that the code is free of vulnerabilities, backdoors and other potentially harmful functions.
By contrast, cryptocurrency networks are described as trustless because all code is freely available and open-source. Anyone can inspect the software's code and know for certain all its functions. Developers can also build their own software implementations based on open standards and already-published code for additional peace of mind.