Draw
Randomly decide whether to fill a cell with black colour.
Logic: Each time the cell is about to be drawn a "dice is rolled" to decide whether to do it.
The chance of filling the cell depends on the Probability value—the higher the probability the bigger the chance that the cell will be drawn.
Note: With the probability of 50% it doesn’t mean that half of the cells will always be filled. Sometimes it will be 30% and sometimes 80%. The bigger the sample (in this case the grid), the chance of filling cells equally rises. This phenomenon is described by The Law of Large Numbers.