The genesis block is the first-ever block in the blockchain, which is often hardcoded right into the application’s software. The blockchain comprises multiple blocks that are often linked together, forming a chain.
Thus, each block in the network contains referential data from the previous block, to check its legitimacy. So, the genesis block is the first block, often referred to as Block 0 or Block 1 for a new blockchain.
What makes the genesis block truly unique is that. It is the only block within the blockchain that doesn’t refer to a predecessor block. For most of the cases, the first mining rewards remain unspendable with the genesis block.