I mean this icon here, in the bottom right system tray thing:
Sometimes when a laptop / tablet / whatever is plugged in, it animates. Sometimes it just sits there like this. In both cases, it appears to be charging just fine.
There's no obvious pattern that I can see in when it animates and when it doesn't, no obvious difference in what it says when moused over.
I've tried searching for information on this and just found a reddit thread full of speculation and contradictory answers. One user speculates that the animation might be the fallback if Windows hasn't yet figured out how full the battery is, but other users give contradictory suggestions.
What's Windows telling me if the icon starts or stops animating?
I'm on Windows 8.1 but I think it's like this in other versions too.