The block that paved way for every other blocks being mined today is the genesis block. Genesis block is also known as Block 0. It was the first bitcoin block that was mined when bitcoin was invented in 2009 by the unknown renowned cryptographer, Satoshi Nakamoto. Genesis block was what other blocks till date were built on. It was mined for a period of six days, the most appropriate explanation is to perfect the blocks to be built on the genesis block. This should be true as the genesis block would be a part of the system forever.
The first block to be mined has 50bitcoins in it and this cannot be spent. It might be to test the block or it's a bug in the network and till date, it's still a subject of debate. The first message the genesis block carried was "The Times 03/Jan/2009 Chancellor on brink of second bailout for banks" and he never commented on the meaning of the message.
Block 0 is the spinal cord of bitcoin network and all blocks will always find their way to it, it's like the blocks root. If every fork of bitcoin is a road, all roads will lead back to the genesis block.