Scrypt is a memory-hard password key derivation function (KDF) designed by Colin Percival in 2009. Unlike bcrypt, it requires large amounts of both CPU and RAM — making GPU/ASIC brute-force attacks proportionally more expensive.
N is the CPU/memory cost (must be power of 2 — e.g. 16384). r is the block size (typically 8). p is the parallelisation factor (typically 1). Doubling N doubles both time and memory cost.
Bcrypt is CPU-only — faster to crack on GPU rigs. Scrypt adds a memory requirement making GPU attacks more expensive. Argon2 (winner of the Password Hashing Competition 2015) is the modern best practice and provides better tuning options than both.