Please fill me in as a non-parent : what was actually up when the baby cried like that?

There's a difference in many of the cries: Boredom, hunger, pain, cold, heat, exhaustion, loneliness, no reason at all, fear...and, once, a hair tourniquet.

A hair tourniquet is formed when a strand of hair lost in a onesie or sock wraps itself around a toe, penis, or finger and the appendage swells, cutting off its own circulation.

It's terrifying to blearily try to diagnose the issue and they just don't stop. Eventually we undid the swaddling and took off the sock and found the purple toe, removed the hair, and then there was a change to the screaming as skin pinch went away and the feeling returned to it, but that was good news.

Not the OP but you get tuned into this pretty quick going through the normal gamut of kid issues. The _worst_ cry I've heard our son do was when he had a double ear infection, it was immediately obvious from the shrill tone that he was in pain and needed to go the doctor. There is also a slightly less desperate one where he's stuck in something (usually leg/arm in the crib, not really stuck but can't yet reason out how to fix the problem).

Double. Oof. That’s no fun.

Very early on we switched from an audio monitor to a video monitor specifically so we could answer this question without panicked running across the house.

It’s really hard to generalize what an alarm cry could mean. In my experience they were never anything outwardly physical. Both of our kids had alarm cries that we associated with waking up from dreams, for example.

For us, our daughter got sick and vomited everywhere once. Of course, the baby has no idea what's happening, so it's scary as well as uncomfortable.

That was a cry I remember.

Our son at ~8 months had managed to get his leg in between the slats in the crib past his knee. I agree with OP -- this was instantly distinguishable from a normal "pick me up" cry.

It's so weird, with COVID time at home, it's almost like I've picked up on my 18 month old's language. All his little noises and cries absolutely have meaning, they're just not in a language I understand. I can't wait until he is able to use words though, it will make it a lot easier!

Maybe he thinks, “I wish this large baby would speak my language.”?

I have a 6 month old going to sleep in my arms... worst cry was probably yesterday with his biggest ever, smelliest poo. The joys of weining.

It's usually either outright pain (examples from our child: wasp sting, finger stuck between the valve handle spring of a garden hose, stomach pain, ...), nightmares or the feeling of being stuck somewhere (examples from our child: arm / leg stuck in the lattice of the baby bed, leg being caught in a shoe rack after climbing on it, ...).