It’s for a cable lock.
Caps lock!
Pretty sure it’s for locking the laptop in place, like to a desk or something.
Is it a Kensington lock?
It’s a lock slot, so you can secure your laptop, like to a table.
It’s for a Kensington lock, look it up.
It’s the K-slot… or the Kensington lock slot.
Most workplace devices have these. But I’ve even seen them on home items. I’ve got NAS units, KVMs, even a monitor with a Kensington lock slot. They’re useful for keeping stuff secured but a bit annoying to set up, honestly.
I think the thing on the left is a vent for heat, and the buttons up top help with typing.
It’s a USB-K port, confusing people while doubling as a security thing!
That’s a USB-D connector. Should work with USB-C… just push real hard until it clicks. 
Wren said:
That’s a USB-D connector. Should work with USB-C… just push real hard until it clicks.
A bit mischievous, don’t you think? Messing with hardware like that isn’t good!
Honestly, I was curious about that slot myself. Good to finally know!
It’s a dust collector.
We call it a ‘radiator lock’ in Poland.
Nobody knows… you’re either part of the mystery or stay out of it!
This is one of those things that even tech-savvy folks might not know about. 
It’s a spot where you can attach a Kensington lock to keep the device from being stolen.
Is this the K slot?
Had my laptop for years and never used it once.