The fundamental counting principle you speak of has to do with permutations, and events such as these:
College Algebra Tutorial on The Fundamental Counting Principle
Say we have 3 types of car colors, red, black, and blue. We also have 2 cars that we can paint any 1 color we want. What are the number of different ways we can do this?
Each car can have 3 colors, and each car color is an independent event from the other car color. Therefore, we have 3 * 3 = 9 possible car color combos for the 2 cars.
Another item of this principle is replacement or no replacement. Your son may experience this in a later lesson. For example, say we have a standard deck of 52 cards. If we pick a card, say a King, there are 51 cards left, and only 3 kings left for our next pick, whereas, before we picked our first card, there was a full deck of 52 cards, and 4 Kings available to draw.
Make sense so far?
If you have real world examples in your son's textbook, folks on the forum can describe this in more detail when they answer the question.