DEFINITION
The Genesis block of any blockchain refers to the very first and inaugural block of that blockchain which is not always making reference to any previous block because it is always the very first block that was mined in that blockchain.
This kind of block is usually labelled in modern versions of Bitcoin as the block 0, but the previous versions usually label it as block 1.
The Genesis block is a very special block because it represents the beginning of that blockchain and all other the blocks that make up that blockchain are layered on the Genesis block. In most of the blockchains like Bitcoin, the Genesis block is referenced in all other blocks in the blockchain and all those blocks are always making use of the template of the Genesis block.

Source

As earlier stated, the Genesis block is always named as block 0 and this seemingly shows that there no data was previously processed before the Genesis block was created, the Genesis block has Its own unique hash just like other blocks.
This unique hash of the Genesis block is always added to the new transactions in the new blocks in order to create or generate another unique hash for the new block, this process is always repeated for all other news blocks that are to be added to that particular blockchain.

SIGNIFICANCES OF THE GENESIS BLOCK.

• A Generic Model for other blocks in the blockchain: The Genesis block of any blockchain is like a generic model for all other blocks that will be added to that particular blockchain. They all follow the template or pattern that was used by the Genesis block. All the features of the Genesis block are always present in all other blocks.

• It gives rise to other blocks: As long as blockchain technology is concerned, the Genesis block is always the first block to be created or mined in a blockchain. So the creation of the Genesis block is always giving birth to the other blocks. It is the first block and other blocks are then added to it to make it a chain.

• An invitation for miners: Every new blockchain is launched or started with the creation of the Genesis block, and that is always like an invitation to the miners because they will now be aware that a new blockchain has hit the ground running and they might want to jump in the trend.

Why is the Genesis block needed

One of the features of the Genesis block is that it shows when a blockchain began, it is like the first ring that a blockchain must have before all other blocks can join for it to become a chain.
If there was no genesis block, it would have been difficult for miners to trust a certain blockchain because the initial status of the blockchain will not beade visible for the miners to explore. But with the availability of the Genesis block, it is easy for the miners to see some data of such block like the timestamp, transaction fee, block height, block difficulty and so on.

THE BITCOIN GENESIS BLOCK

For every Blockchain, there must be a Genesis block, so the Bitcoin blockchain is not an exception. As a matter of fact, it's own genesis block was the very first and most popular Genesis block which was mined on the third day of January in the year 2009 by someone who used the name Satoshi Namatoko.
The creation of this block was rewarded with 50BTC and it is uncertain whether the reward was mined or not.

Note: The only transaction on the Bitcoin Genesis block is the coinbase transaction of 50 BTC.
Output wallet of coinbase transaction:

The transaction was sent to a wallet with the address; 1A1zP1eP5QGefi2DMPTfTL5SLmv7DivfNa
From this other screenshot below, it can be seen that other people have been sending some Bitcoin into that wallet.

Issues with the Bitcoin Genesis block

The Genesis block of Bitcoin have been surrounded by many uncertainties, and one of those uncertainties is the code which makes the block to be completely untradable, it is not known if that was done intentionally or it was a mistake because the developer (Namatoko) did not make any comments about it.

As a matter of fact, many people see the coinbase transaction of the Bitcoin Genesis block to be a mere wierd special case that is contained in the code because the recent Bitcoin system is now handling the databases of the block/transaction in a way that is different from the original system.

A website address was also written into the code of the Bitcoin Genesis block and the block points to that website address, but the web link is always displaying an error message when activated.

The secret message

If the raw data of the Bitcoin Genesis block is converted to the raw HEX version, a message will be seen and it reads;

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

The message is exactly as the headline of an article that was written on The Times newspaper on that same day that the Genesis block of Bitcoin was created.
The article on the newspaper was talking about the failure of the British government to stabilize the economy of the area, many people believe that Namatoko was making a reference to the article as an attempt to hint on how Bitcoin will bring a better financial situation.