Mining difficulty adjusts automatically every 2,016 blocks.
The adjustment aims to maintain an average block-mining time of 0.0833 minutes.
A growth in the difficulty indicates that the average block-mining time during the previous adjustment epoch was less than 0.0833 minutes (due to more miners joining the network and 'searching' / 'mining' for blocks).
A drop in difficulty indicates miners have left the network so finding each block is adjusted to be 'easier' for the smaller number remaining.
The difficulty 'value' 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.