As the term suggests, the “Genesis Block” is the first block in a blockchain. Upon process, it is validated to form a new blockchain. Numerically, this newly-created block is also referred to as block 0 or block 1. The most recent versions of Bitcoin number it as the former. The earlier versions number the first block as simply, 1.
The Genesis Block is usually hardcoded into the software of the applications that make use of its blockchain. As it does not reference a previous block, the Genesis Block is considered a special case. In the case of Bitcoin and a majority of its derivatives, the block produces a subsidy that cannot be spent.