Mining difficulty adjusts automatically every 2,016 blocks.
The adjustment aims to maintain an average block-mining time of 10 minutes.
An increase in the difficulty indicates that the average block-mining time during the previous adjustment epoch was less than 10 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) - e.g. blocks in BTC mainnet epoch 308 are over 16 trillion times harder to mine than those in epoch 0.