Mining difficulty is defined as a measure of how hard it is to maintain and add to the blockchain.
With bitcoin, users are required to own a computer that runs the blockchain program and compete to solve a complex math problem. The first miner to solve this problem creates a new block and typically, this process happens every 10 minutes.
However, the more computers competing to solve the math problem, the quicker blocks will be discovered. To keep block creation at a fixed speed, every 2,016 blocks (every 2 weeks), mining difficulty is adjusted.
As mining difficulty increases, the amount of work required to create a new block and earn the reward increases.