If I asked you how many heads you would get if you flipped a coin 200 times, you would probably say that you'll get 100.
This is because the probability of getting a head is 1/2 so you should get 1/2 of the 100 as heads.
Unfortunately it's not quite that simple. The probability you have been looking at so far is theoretical probability which is what should happen.
We need to look at what happens when you actually do this, if you actually flipped a coin 200 time or rolled a dice 300 time.
This is called Experimental Probability (and sometime Relative frequency  These mean exactly the same thing).
How to find experimental probability.
Experimental Probability = 

In this, the successful trials is how many times you get what you are looking for and the total trials is how many times you do the experiment.
Example: A flip a coin 200 times and get 90 heads. What is the probability my next flip will be a head?
We can now just bang the numbers into the formula.
Experimental Probability = 

This means the probability of my next flip being a head is 9/20 (Don't forget to cancel down)
The link between theoretical and experimental probability.
We now know that theoretical probability is what should happen and experimental probability is what does happen.
The thing you need to know here is that...
The more times you do an experiment, the closer the experimental probability will get to the theoretical probability.
The way they test your understanding of this is through asking a questions where one person does an experiment more times than another. Which one will be more accurate? Always the one who has done more.