Apple has told developers that it has extended the deadline for sandboxing your apps on the Mac App Store from March 1 to June 1 to provide “enough time to take advantage of new sandboxing entitlements available in OS X 10.7.3 and new APIs in Xcode 4.3.”
A sandbox is a security mechanism for separating running programs. Apple is requiring it for Mac App Store apps. The company says sandboxing protects the system by limiting the kinds of things an application can do, such as accessing files on disk or resources over the network.