The Block Height is like the page number of a block in the Bitcoin ledger - it's the number of blocks that came before it in the chain. It's a way of keeping track of where a particular block sits in the grand scheme of things.
Example
"The Genesis Block has a Block Height of 0, since it's the very first block in the chain."
"As of today, the Bitcoinblockchain is over 700,000 blocks tall, and it's growing taller every day!"
