When transactions occur on the blockchain network, blocks are responsible for storing transactions that happen everytime. And during mining transactions, miners compete to verify transactions in already created blocks and create new blocks. Any miner that create these new blocks is rewarded with blocks rewards in form of that cryptocurrency in this case bitcoin and the miner block reward is written as the first transaction on this new block. These new blocks form chain with already created blocks.
But, in the event where two miners create block at a similar time, one is added by nodes on the blockchain while the other is not added. The block that is not added by other nodes on the blockchain network is referred to as an orphan block. Although orphan blocks are valid blocks, they are not added only because of their lower proof of work(POW). Orphan blocks are usually typical with bitcoin blockchain.