A pseudorandom number generator (PRNG) is software that produces numbers that appear random but are actually generated from an algorithm. If the seed or design is weak, the output can be repeated or predicted. That makes the difference between a useful random source and one an attacker can model.
PRNGs matter in cyber security because they are often used to create session tokens, encryption keys, nonces, media keys, and other values that must not be guessable. Attackers look for time-based seeds, reused seeds, or low-entropy inputs, then try to reproduce the sequence and derive sensitive secrets. Defenders use well-reviewed cryptographic PRNGs, strong seeding, and sufficient entropy so outputs stay unpredictable. In devices such as cameras and routers, weak PRNGs can expose administrative or streaming channels even when the rest of the system seems protected.