The challenge creates a file as swap space, so that some of the memory will be putted into this file when physical memory is not enough and will be then fetched from this file when being used again. The problem is this file is writable by any user. Therefore, when privileged memory like code of a root process is putted into this swap space, we can actually tamper it to our shellcode, so when it is executed again, we can execute arbitrary code in root privilege.