When you toss a coin, there are only two possible outcomes, heads or tails. In the applet below you can change the probability of getting heads by entering a new value and clicking Reset. Set the number of tosses and click the Toss button to randomly select heads or tails. The heads/tails bar shows you the cumulative totals. The graph shows this total after each toss.
Notice how the proportion of tosses that produce heads can be quite variable at first, but will eventually settle down to the true probability.