Can someone explain this word to me?
It's a synonym to "aber" (eng: but), though it's not as much used in speech especially colloquial speech. It's not exactly outdated, but it sounds a bit posher/educated, something like that, it's difficult to explain. Maybe it's comparable to the use of "however".
So while I don't use it as much with friends as "aber", I use it instead of "aber" when writing an essay for example.