I never know whether power reserve means running time or something else

It's the running time on a full wind, basically. How long the watch keeps ticking after you take it off or stop winding. Not a 'reserve tank' like a car. Skeleton autos usually list it too, same idea.

Yeah, what he said. On an automatic it's the power reserve when fully wound, so if you don't wear it for that long it stops. Some skeletons have a power reserve indicator on the dial which is kind of cool visually, but not all.

Don't feel dumb, I asked the same thing last year. It's literally just how many hours it runs. My skeleton auto is like 38h which is fine for daily wear but dies over the weekend if I leave it.

Exactly. The indicator is just a gauge. Nice to have on a skeleton since you can see the mechanism anyway, but not necessary.

One thing to watch out for: some cheap skeleton watches exaggerate power reserve numbers. If it's a no-name movement, take the spec with a grain of salt. Not that it matters much if you wear it daily.

Also, if you're looking at a skeleton with a power reserve indicator, make sure it's actually functional and not just decorative. I've seen some where it's just painted on. Ridiculous.

Painted on? That's wild. But honestly with skeleton watches, half the appeal is seeing the gears, so a fake indicator would bug me.