The genesis block is the first ever block recorded on its respective blockchain network, also occasionally referred to as Block 0 or Block 1.
When a block is broadcasted to the blockchain, it references the previous block. However, in the case of the genesis block, there is no previous block to reference.
Because there is no previous block to reference, genesis blocks are generally hardcoded into the software.
In terms of Bitcoin, the genesis block was on the 3rd of January 2009. This block had a reward of 50 BTC that is forever unspendable. It is unknown as to whether the genesis block’s reward was intentionally unspendable as there was no explanation regarding this from Satoshi Nakamoto.
The hash of the genesis block (000000000019d6689c085ae165831e934ff763ae46a2a6c172b3f1b60a8ce26f) is unique as it contains two additional leading hex zeros than required by other early blocks.
The 50 BTC reward from the genesis block was sent to the address, 1A1zP1eP5QGefi2DMPTfTL5SLmv7DivfNa and, as of January 3rd, 2019, this address contains a total of 66.912 BTC and has received a total of 1486 transactions. The block reward of 50 BTC will remain unspendable forever and the people that have sent their Bitcoin to this address will never be able to retrieve the funds that they sent.
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.