Explanation:

Probability is a way of expressing knowledge or belief that an event will occur or has occurred.

http://en.wikipedia.org/wiki/Probability

Frequency probability is the interpretation of probability that defines an event's probability as the limit of its relative frequency in a large number of trials. 

Frequentists talk about probabilities only when dealing with well-defined random experiments. The set of all possible outcomes of a random experiment is called the sample space of the experiment. An event is defined as a particular subset of the sample space that you want to consider. For any event only one of two possibilities can happen; it occurs or it does not occur. The relative frequency of occurrence of an event, in a number of repetitions of the experiment, is a measure of the probability of that event.

Thus, if n_t is the total number of trials and n_x is the number of trials where the event x occurred, the probability P(x) of the event occurring will be approximated by the relative frequency as follows:

  .

(Our solved example in mathguru.com uses this concept)

http://en.wikipedia.org/wiki/Frequency_probability

The above explanation is copied from Wikipedia, the free encyclopedia and is remixed as allowed under the Creative Commons Attribution- ShareAlike 3.0 Unported License.