Height counts blocks from the genesis-block at zero. It is how protocols schedule events without relying on wall-clock time, because every node agrees on height while clocks drift.
Scheduled changes are almost always written as heights. The Bitcoin halving happens every 210,000 blocks, not on a calendar date, which is why the estimated date moves by days as block production runs slightly fast or slow.
Example: if a chain is at height 840,000 and the next scheduled change is at 1,050,000 with a ten-minute block-time, that is 210,000 blocks, about 1,458 days or just under four years.
Related: block, genesis-block, block-time, halving