Block height is the number of blocks between this specific block and the genesis block (first block) on the chain. A genesis block will always have a height of zero because nothing precedes it.
It’s a metric used to create a bearing on time in the programming world as well as a few other functions such as maintaining counter-party and betting in the crypto world. Considering that a new Bitcoin block is made every 10 minutes, you can work out certain time related pieces of information if you have the total length of the chain.