What problem does a salt solve, and how does BCryptPasswordEncoder handle it?

Without a salt, two users with the identical password produce identical hashes, and attackers can precompute a rainbow table mapping common passwords to their hashes once, then instantly reverse any matching leaked hash. A salt is random, unique-per-password data mixed in before hashing, so identical passwords produce different hashes. BCryptPasswordEncoder generates and embeds a random salt automatically inside the resulting hash string, so you never manage salts manually.