This read-only integer reports memory that was previously swapped out and has been swapped back in, but is still also present in the swap file, meaning it can be discarded again without further disk I/O if memory is needed. An admin interprets a nonzero value here as a sign the kernel is efficiently avoiding redundant swap writes for pages it has already written out once. For example, a system under moderate memory pressure showing significant swap-cache would tell an admin the kernel is reusing prior swap writes rather than re-swapping the same data from scratch.