Probability is the measure of the likelihood that an event will occur. It is quantified as a number between 0 and 1, where 0 indicates impossibility and 1 indicates certainty. A simple example is the tossing of a coin that has two sides: head and tail. We can describe the probability of this event in terms of the observed outcomes or the expected results.
For a "fair" coin, the probability of head equals the probability of tail. However, for an "unfair" or "weighted" coin the two outcomes are not equally likely. Change the "weight" of the coin by dragging and dropping the expected probability and see how this affects the observed outcomes.