A Wordlist is a set of 2048 words used to encode private keys (or other binary data) that is easier for humans to write down, transcribe and dictate.
The BIP-39 standard includes several checksum bits, depending on the size of the mnemonic phrase.
A mnemonic phrase may be 12, 15, 18, 21 or 24 words long. For most purposes 12 word mnemonics should be used, as including additional words increases the difficulty and potential for mistakes and does not offer any effective improvement on security.
There are a variety of BIP-39 Wordlists for different languages, but for maximal compatibility, the English Wordlist is recommended.