A genesis block denotes the origins of a blockchain network.
The genesis block is the birth of a new decentralized network, and in essence, its birthday.
For instance, the Bitcoin genesis block was mined by Satoshi on January 3, 2009. Satoshi even embedded a message in the genesis block which highlights the reason why he created the cryptocurrency: “The Times 03/Jan/2009 Chancellor on brink of second bailout for banks.”
“Imagine mining the Bitcoin genesis block? It would have been so easy and you would have received 50 BTC!”