What "Bits of Entropy" Actually Measures
Password entropy quantifies how many guesses, on average, a brute-force attacker would need to find a given password — expressed in bits, where each additional bit doubles the search space. Entropy depends on two things: the size of the character pool used (uppercase, lowercase, digits, symbols each add possibilities) and the password's length, with length mattering far more than pool size for any reasonably long password, since length compounds exponentially while pool size only scales the base of that exponent.
This is exactly why "correct horse battery staple" (a long passphrase of common words) can have higher actual entropy than a short, symbol-heavy password that looks more "complex" at a glance — length dominates the calculation. A password's real-world crackability also depends on whether it resists dictionary and pattern-based attacks, not just raw character-pool entropy — a password using every character type but following a predictable pattern (like a keyboard walk) is weaker in practice than its theoretical entropy score suggests.
Entropy is a useful comparative metric, but it's a mathematical upper bound on brute-force difficulty, not a guarantee against smarter, pattern-aware cracking techniques.