Pin the folders you use most so they're always one click or tap away: as tabs in the header on desktop, and in the bar at the bottom of your phone. You can also drag the folders in the sidebar or menu into the order you like. Your pins and your folder order sync across your devices.
Steps
Desktop
To pin a folder:
Hover over a folder in the sidebar. This works for an account's own folders and labels too.
Click the pin icon (Pin to header). The folder appears as a tab in the header.
Drag the tabs in the header to change their order.
To unpin it, hover over the folder in the sidebar and click the pin again (Unpin from header).
To reorder the sidebar, drag a folder up or down. Drop it below More to tuck it away.
iPhone, iPad & Android
To pin a folder:
Tap ⇅ in the bar at the bottom of your mail list.
Tap Edit pinned labels at the end of the list.
Tap ⊕ next to a folder to pin it, or ⊖ to unpin it. Drag a pinned folder by its handle to change the order.
Tap Done.
The first 4 pins sit in the bottom bar. The rest are in the ⇅ list.
To reorder the menu:
Open the menu and touch and hold one of the folders at the top.
Drag folders above or below the More divider.
Tap Done.
Good to know
The default pins are Inbox, Unread, Starred and Recap. On a phone, at least one folder always stays pinned.
Folders you move below More are still there. Click or tap More to show them.
On a phone, you can reorder the menu only when two or more accounts are in the unified inbox.