BIP39 was a BIP which standardized the way crypto wallets generate private keys. It introduced seed phrases, sometimes called mnemonic phrases, making it easier for users to write down and remember their private keys. It introduced the 2048 words that are used to create a mnemonic phrase (usually 12-24 words long). With each word having 2048 unique words which can be used multiple times, this creates (in a 24 word seed phrase) mnemonic possibilities. In other words, the chances of someone guessing your address are virtually impossible.
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