This is the narrow, wet part just above the lower lash line. You can use a creamy eyeliner pencil here to define and bring out the whites of eyes.
It does tend to come off though, even if you use a waterproof eyeliner pencil. So you will have to keep reapplying at intervals. And check that it hasn't migrated to the inner corner!
Tip: Never use liquid eyeliner on the water line or tight line. One use a soft, creamy eyeliner pencil, or you'll be crying for days!