It's a Chrome extension that adds a "Show Inbox" / "Hide Inbox" toggle button to your Gmail. The inbox is toggled off by default – when you load Gmail, you don't see your inbox right away.
This is nice because it lets you point to your email and compose a new message or search your archive without having to view your inbox as an intermediate step.
It also adds a little more friction to checking your inbox, which is good.
Inbox When Ready takes about two minutes to install & set up. Recommended, if you use Chrome & Gmail.
All credit to Peter Hartree, who made this lovely app.
Also in the "make your tech work for you, not the other way around" arsenal: