In general, the genesis block of any blockchain is the very first block in the blockchain.

Specifically, the Genesis Block is often referred to as the first block of Bitcoin to be mined. 

The Genesis Block was first created in 2009 by Satoshi Nakamoto -- the mysterious and still anonymous creator of Bitcoin. The entire Bitcoin blockchain is built on top of that very first genesis block. 

Those are some of the reasons why Bitcoiners and Bitcoin fans hold the Genesis Block in reverence -- similar to how they feel about Satoshi Nakamoto. They feel reverence so much so that certain devotees have even donated small amounts of bitcoin (BTC) to the Genesis Block -- by way of some type of tribute to Satoshi Nakamoto. This must be a kind of sacrifice because any coin moved into the Genesis Block will stay there forever -- since no bitcoins can ever move from that first block. 

The original Bitcoin client could not find the first 50-BTC transaction in its database -- this meant that any spending transactions from that block will get rejected. Many people wonder why Satoshi Nakamoto did this. Was it a mistake? But most believe that it was intentional since the bitcoin code for the Genesis Block and more is precise and perfect.  We will never know.

The Genesis Block also holds a secret message Nakamoto put into the Block's raw data: "The Times 03/Jan/2009 Chancellor on brink of second bailout for banks."

It’s a headline for an article in the January 3, 2009 edition of The [London] Times about the British government's failure to stimulate the economy following the 2007–08 financial crisis and how they plan to print even more money for big banks. 

No other comments or meanings exist. Its left up to interpretation and thus most believe that it serves as a mission statement for Bitcoin. The creator of Bitcoin and many others wanted Bitcoin to be different in that regard -- that no one would ever bailout Bitcoin.