It boils down to the higher the difficulty the more hashrate (on average) it will take to find a new block, and conversely the lower the difficulty the less hashrate (again on average) it will take to find a block.
A bit more complex explanation difficulty is to keep the emission rate of new coins being generated close to the design parameters for the coin, in this case ZCash. ZCash is specified to generate new coins at a rate of 12.5 zec (10 for miners and 2.5 for founders) approximately every 2.5 minutes, or around 7200 coins/day. Without an adjustable difficulty miners could simply point more hashpower at the coin and increase the amount of coins generated, in theory to the point all the coins could be generated in a few days. By introducing difficulty into the input parameters, a nonce is required to find the next new valid block, it can use this to automatically adjust the generation rate to stick close to the design goals.
Also check out the definition of Difficulty.