Most programs are created using source code that cannot be viewed by anyone other than specialized professionals who are granted access by the creators. This is not the case with open source programs.
An open source program is one which is created with transparency in mind: the code behind the program may be accessed and edited by anyone interested in doing so.
The advantages of open source programming include the ability to allow users to make desired changes for themselves rather than waiting for the program creators to do so. This, in turn, makes open source programs more flexible and adaptable than other programs.
Of course, open source programs do require that users have a certain level of trust in each other, as they are far more susceptible to hacking. This is because anyone can gain access to the source code, allowing them to study it, understand how it works and then hack into websites that utilize the same open source program. While knowledgeable users could likely detect programs that have been altered for malicious purposes, it does pose a legitimate risk.
Nevertheless, open source programming has its strengths. Because users can modify and distribute it at will, these types of programs are much more customizable than other types of software. WordPress is open source, and this has contributed to its popularity among users.