On Proof of Work (PoW) blockchains, block difficulty refers to how much computation is required to mine coins and verify transactions. When more miners send mining hash to the network, it becomes more challenging to mine blocks, increasing block difficulty. Higher network difficulty increases the network security of PoW blockchains since potential attackers will need higher amounts of expensive hashing power to potentially mount a 51% or double spend attack.
NFT Term
Block Difficulty
Definition of Block Difficulty from the NFTguild.com glossary.

