MetaMask provides you with a unique 12-word Seed Phrase / Secret Recovery Phrase on the very first launch. If you did not write it down or lost it, you can log into MetaMask and Reveal your Seed Phrase / Secret Recovery Phrase.
DO NOT share this phrase with anyone! These words can be used to steal all your accounts. You can't edit/change your Seed Phrase / Secret Recovery Phrase.
If you are unable to log back into MetaMask, due to forgetting your password, there is no way to recover the phrase. However, if you remember your password, you should be able to recover the phrase, if you have a copy of the vault data.
You can attempt to find your vault data (either locally on your computer or on a backup of the computer) using these instructions:
To better communicate the importance of never sharing your Seed Phrase, we've changed the name of the phrase to "Secret Recovery Phrase". Learn more here.