Meaning
Straight-line mouth, dot eyes, no expression at all. The neutral face is the emoji equivalent of a blank stare: unimpressed, unamused, quietly done with something, or simply having no reaction.
It is great for deadpan humor and mild disappointment: “The ‘large’ fries came with nine fries in them 😐” It also signals awkward silence when a joke falls flat. Sent as a full reply to someone’s message, though, it can feel cold or annoyed, so with people who might take it personally, add a word or two.