Diceware is a method for generating passphrases, passwords, and other cryptography, like Bitcoin seeds, using ordinary dice.
When creating each word in the passphrase, five rolls of a six-sided die are needed. Each number rolled is assembled as a five-digit number, corresponding to a word in a cryptographic word list. For example, if you rolled 1, 3, 4, 2, and 6 for the first word, you would look up the word "teach" in the word list.
In Bitcoin, diceware is used to generate a Bitcoin wallet seed. A Bitcoin wallet seed is a string of 128 or 256 bits that is used to create all of the private keys for a Bitcoin wallet. Additionally, this seed can also be used to recover the wallet in case the private keys are misplaced or stolen.
Note, remember never to share your seed with anyone, as it would give them access to your Bitcoin wallet.
Read more about Bitcoin wallet safety and privacy here.