Mining difficulty adjusts automatically with every block (ASERT, continuous adjustment).
This page samples one difficulty value every 1,152 blocks (~1 day) to keep the charts responsive.
The adjustment aims to maintain an average block-mining time of 1.25 minutes.
An increase in the difficulty indicates that the average block-mining time during the previous adjustment epoch was less than 1.25 minutes (due to more miners joining the network and searching for blocks).
A decrease in difficulty indicates miners have left the network so finding each block is adjusted to be easier for the smaller number remaining.
The numeric Difficulty is a multiple of the difficulty of finding the easiest block (Block #0).