Mining difficulty refers to the difficulty of a given cryptocurrency and how hard it is to find the right hash for the new block. It is a measuring unit used to determine the difficulty of solving a cryptographic puzzle at the point.
Based on the total number of miners in a given network, the mining difficulty can increase and decrease over time. The difficulty also increases to keep the target block time stable, which is an integral part of mining altogether.
In the case of Bitcoin, the average block time is somewhere around 10 minutes. In order to keep up with the level, the mining difficulty also increases accordingly. These digital currencies are mined via a mining pool, allowing them to combine their hash rates to achieve block rewards.