The human part means you can succumb if you've reached shock stage 50 and you're under 60% total health (where get_total_health is your max health minus fire, brute, tox, oxy, and brain damage). Humans usually have 200 max health, but some species (e.g. monkeys) have different amounts.
Shock stage itself is essentially just a representation of how long your character's been in a certain amount of pain (see carbon/human/life.dm, the threshold depends on the amount of shock you already have). At around 30 shock, you start randomly stuttering. You're locked to shock stage 61 and above when you're in asystole, and that's also about the point when you start getting weakened randomly.
The robot part is just enabling succumb when you're at 33% health or below. At ~70% a robot's armor is knocked out, at 60% health a robot's voicebox is knocked out, and at the 33% threshold you've usually either had your power cell knocked out (sleeping you) or had your actuators knocked out (effectively locking you down).