The genesis block is the first ever block recorded on its respective blockchain network, also occasionally referred to as Block 0 or Block 1.
Within the Bitcoin genesis block, a message was left by Satoshi, forever written in history. Within the coinbase parameter, along with the regular block data, the following message was included (a headline from The Times).
The Times 03/Jan/2009 Chancellor on brink of second bailout for banks
This message was likely included as a timestamp to prove that the block was not created before the 3rd of January. However, the chosen headline also has strong relevance to the reason Bitcoin was created in the first place.
After the genesis block of Bitcoin, the next block on the blockchain had a timestamp recorded six days later.
Block 0: 2009-01-03 18:15:05
Block 1: 2009-01-09 02:54:25
The exact reason behind this is currently unknown, but it is hypothesized that Satoshi either took a break or planned this to correlate with the Bible's account of the world being created in 6 days.