Genesis Block
In short: The genesis block is the first block of a blockchain network, also known as block 0. It is the only block that does not refer back to an earlier one, and…
The genesis block is the first block of a blockchain network, also known as block 0. It is the only block that does not refer back to an earlier one, and its contents are hard-coded into the network's node software.
Why it matters
Every later block contains the hash of the block before it, so the entire chain can be traced back, link by link, to the genesis block. Because all nodes start from the same hard-coded block, it anchors the network's history and determines which chain a node belongs to; networks with different genesis blocks, such as a mainnet and its testnet, cannot share blocks. For many projects, the genesis block also records the initial distribution of coins.
Example
Bitcoin's genesis block was created by Satoshi Nakamoto. Its first transaction carries the text of a newspaper headline about a bank bailout, widely read as a comment on the traditional financial system. The 50 BTC reward in that block can never be spent, because of a quirk in how the original software handled it.