Copyright is a legal concept enacted by most governments to give creators of original works exclusive rights to it. This is usually time-limited, and is intended to allow the creator to be compensated for their work.
Copyright law gives exclusivity of the rights to copy, distribute and modify a work to its creator for a given period of time, in the interest of getting such a work disclosed to the public.
There are various differences in copyright law across countries, and the frequent advice given on Stack Exchange regarding it is I Am Not A Lawyer (IANAL) - we are freelancers, and given the complexity of copyright law, we are not a sufficiently good resource for asking in-depth questions about copyright.
Since the standard IANAL tagline is to consult a lawyer, a question was posed on Stack Overflow regarding where to find them, and is a suggested resource for finding one.
There are opinions specifically on software copyright, but much more debate can be found in the wider world of copyright as a whole - see the Philosophy of Copyright for both sides of the issue.