Network difficulty is a numerical representation of the current mining difficulty and adjusts, as the hashrate produced by miners fluctuates, in an attempt to have targeted blocktimes of 2 minutes.
If mining hashrate increases, then network difficulty also needs to increase, in order to maintain the 2-minute average for blocktimes. Conversely, if hashrate decreases, then difficulty will also need to adjust down.
Network difficulty adjusts with every block found, and is based on the average blocktimes for the last 60 blocks.
This means that short durations of significant fluctuations in mining hashrate can result in blocks that are found faster (higher hashrate) or slower (lower hashrate) than the 2-minute target blocktime.
It is desirable to minimise large fluctuations in mining hashrate, as this will result in blocktimes that stay fairly close to the 2-minute target.
The current Network Difficulty can always be found in the Block Explorer.