Mining difficulty uses LWMA (Linearly Weighted Moving Average) and recalculates on every block, not at fixed intervals - this chart samples one data point every 144 blocks for readability.
The adjustment aims to maintain an average block-mining time of 10 minutes.
An increase in difficulty means recent blocks were found faster than 10 minutes on average (more hashrate on the network).
A decrease in difficulty means recent blocks were found slower than that (less hashrate on the network).
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.