Genesis Block is the name of the first block of bitcoin ever mined—thus called “Genesis.” It is also referred to as block zero.
The genesis block forms the foundation of the entire Bitcoin network.
The genesis block was mined by Satoshi Nakamoto on 3rd of January 2009, a few months after the release of the Bitcoin whitepaper on October 31, 2008.

The hash of the genesis block (000000000019d6689c085ae165831e934ff763ae46a2a6c172b3f1b60a8ce26f) is unique as it contains a hidden message included by Satoshi Nakamoto.
It includes a headline from The Times of England.

The Times 03/Jan/2009 Chancellor on brink of second bailout for banks

We can verify this message is in the Bitcoin blockchain by interrogating our Bitcoin full node.

$echo -e "$(bitcoin-cli getblock $(bitcoin-cli getblockhash 0) 2 | grep -oP '(?<="coinbase": ")[^"]*' | sed -e 's/../\\x&/g')"

Bitcoin Genesis Block

We can confirm that the message embedded in block zero (Genesis block) correlates with the heading of The Times UK publication of 3rd of January, 2009. Bitcoin Genesis Block