The genesis block (also called Block 0) is the very first block in a blockchain. It serves as the foundation upon which all subsequent blocks are built. Every blockchain has exactly one genesis block, and it is typically hardcoded into the software.
Bitcoin's Genesis Block
Bitcoin's genesis block was mined by Satoshi Nakamoto on January 3, 2009. It contained the famous embedded message: "The Times 03/Jan/2009 Chancellor on brink of second bailout for banks" — a headline from The Times newspaper, widely interpreted as a commentary on the instability of the traditional banking system.
Significance
The genesis block establishes the initial state of the blockchain. It sets the starting parameters — the initial difficulty, the first block reward, and the base for all future block hashes. Unlike all subsequent blocks, the genesis block has no reference to a previous block.
Fun Fact
The 50 BTC reward from Bitcoin's genesis block is unspendable due to a quirk in the original code. Despite this, people have sent additional Bitcoin to the genesis block address as a tribute to Satoshi — these coins are also effectively lost forever.