When this flag is set, the kernel maps a vDSO page into newly created processes and passes its address down to glibc upon exec(). This feature is enabled by default.
vDSO is a virtual DSO (dynamic shared object) exposed by the kernel at some address in every process' memory. It's purpose is to speed up system calls. The mapping address used to be fixed (0xffffe000), but starting with 2.6.18 it's randomized (besides the security implications, this also helps debuggers).