Last November Apple announced to its developer community that all applications distributed through its Mac App Store would require sandboxing. Apple initially set the deadline for this requirement to March 1, but recently moved this deadline back a few months to June 1.
Sandboxing is a method of isolating an application's tasks from those of other applications and the system, by allowing it access to only the resources it is intended to access.
This setup prevents any errors in the program from interfering with resources it was not intended to access. For instance, if a program is built to edit video files and does not use your address book, then it wouldn't normally access your contacts, but bugs in the code might result in it doing so.
Normally that would lead to an unstable work flow or data loss at most. But it is also possible that attackers could exploit such bugs and use them to gain access to any private information that the program can access.
Sandboxing ensures that a program will not access any unnecessary resources, and thereby reduces the potential for security breaches or corruption that might occur if it has bugs in its code.
While Apple initially mandated that all applications in the Mac App Store have sandboxing enabled by this week, according to Kaspersky Lab Apple decided to push the deadline back because of new entitlement options it introduced in OS X 10.7.3 and also new sandboxing APIs in the latest XCode release to get developers up-to-date with Apple's new services such as Gatekeeper. The June 1 deadline gives developers time to update their programs with these features.