The display name is a helpful means of providing identification to the emails you send, in addition to your email address.
Your display name can be added in the Settings of your account.
Prior to setting a display name, your email address is automatically used as a display name in most email clients. If your email address is email@example.com, your display name will be shown to the recipient of the mail as sarahw.
Below you can see what the email address firstname.lastname@example.org displays as when there is no Display Name set:
How the message will appear in Gmail (notice the far left displays “contact” as the display name):
Upon saving a Display Name, your messages will appear under a different name when received by the recipient.
Below you can see what happens when we change the display name for email@example.com to ProtonMail.
Releasing the Custom Domains feature, we also added a possibility to add/create different Display Names for each alias. To learn more about this, please visit this link: https://protonmail.com/support/knowledge-base/display-name-aliases/