BIP39 (Bitcoin Improvement Proposals 39) is a standard used for generating mnemonic phrases. It defines a mnemonic code system that makes it easier for users to remember long strings of random numbers, which are typically used to generate private keys for cryptocurrency wallets, such as Bitcoin wallets. The BIP39 standard was proposed in 2013 and describes a mnemonic code system that converts a longer binary string (usually representing a private key) into a series of common words, making it easier for individuals without technical expertise to remember.

A mnemonic phrase generated according to BIP39 standards usually consists of a sequence of 12 to 24 words drawn from a fixed wordlist, which contains 2048 different words. Each word represents a certain number of bits, and the entire mnemonic phrase collectively represents a larger block of information that can be used to regenerate the user's private key.

The BIP39 standard also includes a checksum concept, meaning that the generated mnemonic phrases are not just random combinations of words but are calculated to ensure that the original data can be correctly recovered even if there are minor errors in inputting the mnemonic phrase. Additionally, users have the option to add an extra "passphrase" to further enhance security.

In summary, BIP39 aims to improve the reliability and security of the generation and recovery process of mnemonic phrases, thereby enabling users to manage and back up their digital assets more securely.