Source code is the instructions that make up an application or utility for the computer in human-readable form. The source code is compiled or interpreted by the computer to run the application.
Access to the source code should be restricted.
Source code can be the main Intellectual Property (IP) of your organization, and thus very valuable. Source code can also be less valuable to the organization but very valuable to attackers, as the source code may contain passwords and other sensitive information, or can be analyzed by a potential attacker for weaknesses that they could exploit.
It is expected that access to the source code is limited to the development team and further limited to members of the team working on that project. There may be many projects for different applications, and not all developers are working on all projects at the same time.
Further access to the source code should be reviewed on a regular basis to ensure that the developer is still working on that project and has not been reassigned or left the organization.