The genesis block is block 0, mined on 3 January 2009. Its coinbase contains a newspaper headline: "The Times 03/Jan/2009 Chancellor on brink of second bailout for banks."
The headline acts as a timestamp, and it points to the reason Bitcoin was built. The genesis block's 50 BTC reward cannot be spent under the consensus rules.
Takeaway: Every block since links back to this one.