The mining difficulty measures how many hashes miners need to perform to find a block. It automatically adjusts every two weeks, depending on how much computational power is involved in verifying transactions. The difficulty plays an essential role in ensuring network security - the higher the difficulty, the more resources attackers would need to take over the blockchain.
As Bitcoin supply crosses 19 million, with only 2 million left, more miners are expected to join the network in the race for mining rewards. The increased interest in mining corresponds with a historically strong accumulation of BTC from both retail and institutional investors.