Keep splitting a number into a product of two factors. However you split it, once only unsplittable primes remain, the result is always the same.
Prime factorization means writing a number as a product of primes (numbers divisible only by 1 and themselves). In the tree diagram, multiplying the two numbers at any branch together always gives back the number above it — and multiplying just the bottom leaves (the primes) together gives back the original number too.