Visitors can enter freely and transact business, but can't go beyond the front counter. Applets from known and trusted sources might be allowed to read or write specific files for instance, while those of more dubious origin would not. For instance, the Java architecture enforces a strict separation between application programs, on the one hand, and system functions, on the other. In traditional platforms, application and system functions tend to intermingle indistinguishably, making it pretty much impossible to cordon off application code in the fashion of the Java sandbox. No amount of tinkering after the fact can provide traditional technologies with the kind of brick-wall security that the Java platform has built in from the start.
Trusted code is given unlimited access to the entire system.
The strength of the Java security model is the way it manages to combine a great deal of flexibility with a high degree of safety. If applets are untrusted, they're placed in the sandbox, where they can run without threat to the system. If they come from a trusted source, on the other hand, they're allowed to use particular system resources according to specific permissions. In no case do they have free reign to go wherever they like and do whatever they please. So the Java platform is not only more secure, it's more robust.