"F_p" means the field of integers modulo p. Fields have addition and multiplication. If p is prime, the integers mod p have nice properties, like every one ha

"F_p" means the field of integers modulo p. Fields have addition and multiplication. If p is prime, the integers mod p have nice properties, like every one has an inverse. For example, F_7 is the finite field of 0,1,2,3,4,5,6. You can add and multiply these (mod 7): 6 + 2 == 1, 2 * 5 == 3. The elements 0 and 1 work as you'd expect to satisfy the usual identities of arithmetic. Additionally, every element except 0 has a multiplicative inverse. 1 * 1 == 1, 2 * 4 == 1, 3 * 5 == 1, and 6 * 6 == 1. To "divide" by 2, you just multiply by 4. Etc.