“Block height” is the number of blocks from the genesis block to the current one.
Blocks are where all Bitcoin transactions are stored and every 10 minutes, a new block is created with new transactions added on top of the previous ones.
For example, 10 minutes after the genesis block was created, a new block was added which would give a block height of 1.
If you were looking for a specific transaction in Bitcoin’s blockchain, you would find it at a specific block height. As more blocks are added with more transactions, the blockchain gets longer and so the block height will increase over time.
The block height always traces back to the genesis block.