The speaker describes shutting people out to avoid being hurt. Closing a door, adding a chain and a lock, becomes a picture of hiding your feelings and acting like nothing bothers you. On the surface the person seems unbothered, but the poem suggests it's a defense: the lock is there "to keep your heart from breaking into pieces."
The mood is lonely and heavy. There is some comfort in the solitude, since the person can cry themselves to sleep and trust only God and themselves. But the poem also shows the cost. Even when they scream, nobody hears, "not even a sound that a ant can hear," a plain, striking way to show how total the silence is. By the end, the protective door has turned into a prison, and the chain that was meant to keep feelings safe now traps them too. The last line, "True feelings lay there," leaves the sense that the real self is still waiting behind the door, unseen.