PostgreSQL is an Open Source project. Open Source by definition means that you can obtain the source code, use the program, and/or modify it freely without the confines of proprietary software. In the database world, Open Source means that you have honest access to benchmarking numbers and performance statistics that companies such as Oracle prohibit. Open Source also means that you are free to modify PostgreSQL to fit your particular needs.
However, there is a misconception that since Open Source is free from distribution restrictions, that it is always free of cost to your company. This is not necessarily the case. It is true that you can, without external cost, download and install Open Source software, but there will always be costs associated with the time and energy your company puts into support and research of the application. As such, if you do not have those resources to spend, there are several commercial vendors and consultants who deal specifically with PostgreSQL.