The Original Meaning: When faced with an unconscious or unresponsive patient, one of the first things an emergency responder does is shine a penlight into each eye, in turn. If the pupils don't contract (are not reactive), or don't react to the same degree (are not equal), that's an indication of head trauma, concussion, and possible brain damage.
When Your Obedient Serpent Says It: It's usually in response to "how are you?", and invoked when the answer is "All Systems Nominal, with cause for optimism."
Because, you know, any day where you don't have brain damage is a victory, right? Any landing you can walk away from.
... I have no idea if this is a glass-half-empty or glass-half-full attitude.