Hi Iris!
It means that you call a call centre and they ask you to wait for a few minutes, but sometimes the waiting lasts for hours. Probably this is what makes people angry. This happened once in an episode of Friends, when Phoebe called a call centre, was asked to wait for a few minutes, but a few hours later she was still on the phone... and didn't want to hang up, because she thought that the waiting might end soon.