Encryption is when you change data with a special encoding process so that the data becomes unrecognizable; it's encrypted. You can then apply a special decoding process and you will get the original data back. By keeping the decoding process a secret, nobody else can recover the original data from the encrypted data.
Mailvelope is a browser add-on for Chrome, Edge and Firefox that securely encrypts your emails with PGP using Disroot webmail. Check this [guide](https://mailvelope.com/en/help) for more details.