Everyone carries some kind of hurt, the speaker says, and most people hide it. The "skin coat" is a nice image: skin becomes something you wear to cover up what's underneath. Nobody really thinks about those bruises until evening, when the sun goes down and things get quiet enough for the pain to show up.
The second half goes deeper. Some people have "deep gashes" that never broke the surface, a knife that went in without piercing the skin. That's a way of describing hurt that leaves no visible mark, which is why the speaker asks, almost confused, "so why did it hurt so bad?" The last line is the heaviest. Waking up feels so unbearable that stapling their eyes shut sounds better than facing the day.
The mood is tired, raw, and a little bleak. The speaker seems to understand this pain from the inside, and the poem reads like an attempt to say that invisible hurt is still real.