An orphan or detached block is a valid block but it’s one that isn’t part of the blockchain. Such blocks are produced when more than one miner puts a block on the blockchain in the same moment. Orphan blocks can also be produced when hackers or attackers are trying to reverse transactions.
To put it simply, an orphan block doesn’t have a known location in the blockchain. This is not the same as a stale block though, which isn’t part of the blockchain at all. Orphan blocks can be a problem for miners because they occur very frequently. Blockchain.info keeps track of orphan blocks on the network and it’s been reported that possibly 1-3 of these orphan blocks are created every day.
In the Bitcoin blockchain, a new block is mined every 10 minutes which means around 144 block rewards will be available regularly available to miners. It also means that in excess of 1% of blocks mined every day will be orphan blocks.
