What is a product of prime factors?

A product of prime factors is a set of prime numbers that multiply to make the requested number. Essentially, when a question asks you to express a number as a 'product of primes', it wants you to break the number down into purely prime numbers. For example, to break 50 down into a product of primes, we would first break 50 down into 2 x 25: 2 is our first prime number and therefore we do not have to break that down further. 25 is not a prime number and therefore we break that down into 5 x 5 - 5 is a prime number and we have two 5s. The prime numbers we have found are 2, 5 and 5. Therefore 50 as a product of prime factors would be expressed as 2 x 5 x 5, or even better, 2 x 5²