You learned modular arithmetic. You understand the mod operator, congruence notation, and the key properties: (a + b) mod m = ((a mod m) + (b mod m)) mod m, and the same for multiplication.
You saw why contest problems ask for answers mod 10^9 + 7, and how to use fast exponentiation to compute huge powers efficiently. You also know how to handle negative results and use modular inverse for division.
Next section: number theory and prime numbers.