DCP-o-matic is free and open-source.
Free means simply that it is free; you can download it without paying, and there are are no catches, clauses on commercial use, requirements for registration, codes, watermarks, ‘pro-editions’, browser tool-bars, adverts nor any such like.
Open source means that you can see how it works and alter it. The only requirement is that if you alter it and redistribute it, you must share your changes.
There is a licence which you can read at your leisure; this provides much more detail.
But, in essence, share and enjoy!