n = the total number of observed cases
f = the number of cases favorable to the phenomenon whose probability is being calculated.
For example, there are 6 sides to a die marked A-F, what is the probability that an A will be rolled? n=6 and f=1. So, the answer is 1/6.
Same die as above, however, after rolling the die for a 1000 times, you note that A came up only 100 times. n=1000 and f=100. So, the answer is 1/10.