A stale block is a digital recording that is complete with information and no longer needs to be recorded or worked on. Because a blockchain is a digital recording and each block is like a page in that recording, new pages are added any time more information needs recorded. If everyone were to record new info at the same time, the blockchain wouldn’t work. So the system lets one person record info at a time. A stale block is a block that has already had info recorded onto it. There is no need for people maintaining the blockchain to try to record more information on it.