What the licence permits
Open source does not mean unrestricted use. A licence says what people may do and what conditions they must follow. Permissive licences such as MIT and Apache generally allow reuse while requiring notices. The GPL is a copyleft licence: people who distribute covered software must make its source code available to recipients, and distributed modified versions stay under GPL terms.
Proprietary software usually keeps its source code private and limits use and sharing more tightly.
Choosing open-source software
Teams consider maintenance, security, support, and whether the licence fits their plans. Even a popular project may depend on a small group of maintainers. Distributing software with open-source components can bring licence obligations, so teams check the terms before release.