To temporarily shut off the thinking part of your brain and focus only at the task at hand, such as eating, playing video games, watching television, or exercising.
Husband: ".........." (Eats dinner while running sleep mode)
Wife: "Whatcha thinking about?"
Husband: "..........Nothing." (Continues eating as he enters sleep mode again)
When you become an option. You are not first pick of friends. When your friend starts dating someone and you are put up on a shelf until they have no other people to hang with.
I don't need you anymore, I'm dating somebody. So I'll put you up on the shelf until I have a need for you. You are my shelf friend.