Block height refers to the number of blocks in a blockchain. Each block added to a blockchain is assigned a unique height, with the first block having a height of 0, the second block having a height of 1, and so on. The current height of the blockchain can be determined by counting the number of blocks that have been added to it.
In the context of Bitcoin, the block height is also commonly used to refer to the number of confirmations a transaction has received. Each block added to the blockchain confirms all of the transactions included in the previous block, and so the more blocks that are added after a particular transaction, the more confirmations that transaction has received. A transaction is typically considered to be secure and irreversible once it has received six or more confirmations.
Block height is also important for determining the difficulty of mining new blocks, as the difficulty is adjusted every 2016 blocks (about 2 weeks) based on the time it took to mine the previous 2016 blocks. A higher block height means that the difficulty is higher and it will take more computational power to mine new blocks.