What is Bitcoin mining difficulty?

How hard it is to mine a block. Bitcoin resets it every 2,016 blocks, about two weeks, so blocks keep coming about every 10 minutes.

If the last 2,016 blocks came faster than every 10 minutes on average, difficulty goes up; if slower, it goes down. Each change is limited to a factor of four either way.

A rise means each hash has a lower chance of meeting the target. It does not directly change the block subsidy or the fees paid by transactions.