What is block height?
A block's number, counting from the genesis block at height 0. The current height is how many blocks the chain has.
Each new block's height is one more than the block before it, so the latest height tells you the chain's length. Explorers use it as an address for a block: block 0 is the genesis block.
A payment's confirmations are the current height minus its block's height, plus one. Schedules like the halving's every 210,000 blocks are set in heights, not dates.