So I spotted this on my brake today, and I don't know what to call it.

It's sorta like a quick-release lever thing? But it seems to adjust the tension on the brake cable. I assume this is meant to be a device to get your wheel out of the brakes while taking it off...?

(Will post pic in a sec, hang on...)

Yes that is a quick release.

I have a road bike with a similar quick release and a SS road bike with the quick release in the brake lever.

There are also quick releases on some brakes systems where you adjust the cable tension on the caliper and you can twist it in and out of release

Now that's just plain clever.

Yup, you use that to take the wheels out. You can also adjust it while riding if you notice your brakes are rubbing on the rim.