The genesis block is the first block of a blockchain. It is hard-coded into the node software rather than mined, because there is nothing to build on before it exists. Every subsequent block references the previous block by hash, so the genesis block is the anchor from which the entire chain grows. If you change the genesis block, you have a completely different chain.
Bitcoin’s genesis block was mined by Satoshi Nakamoto on 3 January 2009 at 18:15:05 UTC. The block contains a single coinbase transaction paying 50 BTC to an address nobody has ever spent from (those 50 BTC are therefore, as a quirk of the implementation, actually unspendable — they cannot be included in any future block’s UTXO set). The block also contains an embedded text field, set by Satoshi, reading:
The Times 03/Jan/2009 Chancellor on brink of second bailout for banks
This is a reference to the actual headline of that day’s London Times, which was reporting on the UK government’s second financial crisis bank rescue. The inclusion serves two purposes. First, it timestamps the block to at least January 3, 2009 — nobody could have mined that block before the headline existed. Second, and more famously, it is read as a political statement: Bitcoin was launched as an alternative to a financial system that needed periodic bailouts to function. Whether Satoshi meant that explicitly or was just making sure the genesis block could not be pre-dated is still debated.
Genesis Blocks on Other Chains
Every blockchain has one. Ethereum’s was mined on 30 July 2015. The genesis block there includes the initial state from the pre-sale — the addresses that bought ether in the crowdsale before launch, with the corresponding balances. Rather than a fresh block with no state, Ethereum’s genesis block is already populated with a substantial ledger. This was the mechanism for distributing the roughly 60 million ETH that had been sold before the network went live.
Testnet genesis blocks are usually less interesting — they are just blocks with some prefunded test addresses — and get replaced every time a testnet is reset. Mainnet genesis blocks, by contrast, are permanent and have a kind of ceremonial status among the communities that care about chain history.
Why You Would Ever Care
The genesis block almost never matters for day-to-day usage. You will probably never interact with it. What matters is that it exists — it is the reference point that makes the chain auditable all the way back to its start. You can trace any Bitcoin, follow any Ethereum address history, or verify any on-chain claim back to a fixed, public, cryptographically linked origin. Trying to do the same in a centralised database is a political and legal exercise. On a blockchain it is just a recursive walk of hash pointers that terminates at the genesis block.