A Bitcoin mining pool difficulty is a measure of how hard it is to find a valid block when mining in a pool. It is an important factor as it affects the mining rewards and the rate at which blocks are found.

Generally, the difficulty is adjusted so that miners find blocks at a rate of one block per 10 minutes. This is known as the block time. When the difficulty is relatively low, the block time can be shorter than 10 minutes, and when the difficulty is high, it can take longer than 10 minutes to find a block.

The difficulty is adjusted by the mining pool, depending on the total hash rate of the miners in the pool. A higher hash rate means it is more likely for a miner to find a block, so the difficulty is increased to keep the block time at 10 minutes.

When the hash rate of the pool increases or decreases, the mining pool adjusts the difficulty accordingly. This ensures that miners are rewarded fairly and that the block time stays consistent.

Ultimately, the difficulty of a mining pool is a very important factor in determining how profitable mining is. If the difficulty is too low, miners will receive too many rewards, and if the difficulty is too high, miners will receive too few rewards. It is important to choose a mining pool with an appropriate difficulty for the desired mining rewards.