“Zero” doesn’t refer to the number of emails in your inbox, but the amount of time your mind is in your inbox. Productivity expert Merlin Mann developed the Inbox Zero technique to prevent your inbox becoming a to-do list. Choose one of five actions for each email: delete, delegate, respond, defer and do. He also advocates not leaving your inbox open and only checking email at certain set times of the day. Here’s Merlin talking about it: https://youtu.be/z9UjeTMb3Yk.
Source: learn.filtered.com