Entropy is the measure of unpredictability in data, and in Bitcoin it determines how difficult a private key is to guess. Wallets convert entropy collected at creation into a seed, which then derives every private key and address the wallet will ever use. Strong entropy makes a key practically impossible to brute-force; weak entropy leaves funds exposed no matter how carefully the wallet is used afterward.
A seed is fixed at the moment of creation, so entropy failures cannot be repaired by software updates. If a wallet was generated from predictable data, the only remedy is creating a new wallet with strong entropy and moving funds to it. Several real-world thefts have traced back to wallets that drew randomness from a single weak source.
Hardware wallets guard against this by mixing multiple independent entropy sources. Blockstream Jade, for example, hashes user interaction timing, CPU counters, uninitialized memory, a built-in hardware random number generator, and entropy from its companion app through SHA-512, so no single weak source can compromise the resulting recovery phrase.