до востребования

What the hell is Poste Restante?

I don't even know what is means in English

It means simply that the letter or parcel is kept/stored* in the post office untill the reciever comes and asks for/takes it away.

When you mark the post до востребования you do not know for sure when it will be taken by the reciever.

I myself always wondered how to say it in English.

*edited

I would say that the term the USPS would use is something like "Hold for Pick -Up." But then, I could be wrong...I don't think it's that common any more, either. I think you'd only really encounter it if someone shipped you too large a package to physically put in the mail-carrier's vehicle -- which doesn't happen very often b/c most people here use UPS, FedEx, or some other private sector delivery service for those kind of things.