The bitcoin network has a global block difficulty that adjusts every 2016 blocks (~2 weeks) based on a target time of 10 minutes per block. Valid blocks must have a hash below this target, therefore difficulty is a measure of how difficult it is to find this hash. As difficulty increases, more hashpower must be added to have the same statistical chance of finding a block. The time between bitcoin blocks can vary dramatically if there is a large increase or decrease in hashpower within this 2 week period.
Source:  BitcoinVisuals nodeLearn More