From the commit message:

    emacs: Make queries used in the all-tags section configurable
    
    This patch adds a customization variable that controls what queries
    are used to construct the all-tags section in notmuch-hello. It allows
    the user to specify a function to construct the query given a tag. It
    also allows hiding tags by returning nil.

Possible uses would be things like displaying the unread count for
tags instead of all messages with that tag and/or hiding uninteresting
tags like "signed" or "encrypted".

-- Daniel