Block height, explained
Block height is the number of a block in the chain, counting up from the first block in 2009.
The first block, called the genesis block, is height 0. Every new block adds one. A new block arrives about every 10 minutes, so roughly 144 blocks are added each day.
Bitcoin schedules its rules by block height, not by calendar dates. The halving happens every 210,000 blocks, and mining difficulty adjusts every 2,016 blocks, about every two weeks. Every computer running Bitcoin agrees on the block count, so the rules switch on at the same moment for everyone.
Some bitcoiners even date their posts by block height instead of the calendar. Satoshi's early code called the chain a "timechain" for this reason.
To see the current height, check a block explorer such as mempool.space.