HOW WILL THE DIFFICULTY LEVEL AFFECT BITCOIN MINING?

A simple answer:

The higher the difficulty rate is, the harder is is to create Bitcoins. There, that was simple

Difficulty is a measure of how difficult it is to find a new block compared to the easiest it can ever be. The difficulty changes every 2016 blocks.

The following is the formula, which calculates the difficulty:

difficulty = maximum_target / current_target
(target is a 256 bit number)