Open source software publishes its source under a licence. You can inspect it, run it, and usually improve it. That is why WordPress, Linux and so many developer tools exist as commons.
It is not the same as “free of charge” and it is not the same as “no rules”. Licences decide whether you must share your changes. Companies use open source to move faster; they still owe security and maintenance.
Use it when you want control and a community. Budget time to update, or you inherited a museum.