Downloads every message currently in your mailbox.
If you want to download your entire mailbox without setting up a POP client, Google provides official free tools for local archiving: Read Gmail messages on other email clients using POP pop free download gmail
Before you can download emails using a POP client, you must enable the feature within your Gmail account settings: Downloads every message currently in your mailbox
Only downloads new messages received after this setting is saved. keep in Inbox
Click the "Forwarding and POP/IMAP" tab. Choose a Download Mode:
Choose what happens to Gmail's copy of the email after it is downloaded (e.g., keep in Inbox, archive, or delete).