Related to cryptocurrencies that use the POW protocol, thus are mine-able. Miners get a specified reward for completing a block ... let's say 10 coins. No matter how many miners are currently using their hashing power to mine the same currency, the reward is still the same and it still takes the same time to fill a block. So in terms of hashing power, the more miners are involved, the more power is needed to complete a block.
So, the higher the current difficulty of a crypto currency is, the more hashing power it takes to fill a block and the lower reward per unit of power is awarded to miners.