The chunk updates readout
Chunk updates is the readout for the number of 1-block spaces which have changed state or content in the last second. This change in state can be caused by the addition or subtraction of blocks from the map, or the rapid filling of a space with a liquid.
Chunk updates can be high right after Map generation, or if a griefer is flooding an underground base in Classic.
Chunk updates could be viewed directly on-screen since Classic. From Alpha onwards, debug tools are hidden by default and the chunk updates readout can be viewed by holding F3.