The center line of keys on QWERTY keyboards.
The letters "asdfghjkl;" are on it.
Your left hand should cover "asdf" and your right "jkl;".
If you'll notice, you have a little bump under your pointer fingers on each hand. (If you're on a Mac than the bumps will be under your middle fingers, not your index)
This makes it easy to find your way back to the correct hand position without looking, should you stray off of it.