With this latest improvement to the Inbox, you can now select multiple conversations at once and perform bulk actions on the selected group. Groups can be entire folders or the results of a search query.
Mark read / unread or archive multiple messages at once by either selecting all with the checkbox at the top of the screen, or selecting messages manually. Note that selecting all will initially select only the conversations visible on the screen. The banner at the top of the inbox will then provide the option to select all the conversations within the group.