Product of prime factors
A composite number is a number which is not prime. We can build any composite number by multiplying prime numbers together - prime numbers are the building blocks for all the other numbers.
e.g. 15 = 3 x 5, and 16 = 2 x 2 x 2 x 2.
To write a number as a product (multiplication) of its prime factors (prime numbers that go into it), we need to divide it up into prime numbers. We can do this in little steps (see the tree diagram in the picture).
Keep going until every branch of the tree ends in a prime number. We then write down all of these prime numbers as a product (multiplied together).
Sometimes an exam question might ask for your answer in index form - this just means you need to simplify your answer using powers.