You learned divisibility rules for 2, 3, 5, 9, 10, and 4. You understand factors, multiples, primes, and composites. You know how to find all divisors of a number in O(√n) time.
You learned the Sieve of Eratosthenes, which finds all primes below n in O(n log log n). You solved three problems: Count Primes, Ugly Number, and Perfect Number.
You also learned about GCD, LCM, coprimality, and the fact that there are infinitely many primes. Next section: modular arithmetic and fast exponentiation.