Three short lines, and almost nothing happens: a hand that was cold is warming up. But the exclamation mark tells us this small change matters a great deal to the speaker. It reads like a quick, surprised announcement, as if the speaker has just noticed the tingle of feeling returning and can't help saying so out loud.
The poem doesn't tell us why the hand was cold. Judging by the wording, we can imagine winter air, though the text never says it. What it does give us is the feeling of relief. Anyone who has come in from the cold and felt their fingers slowly wake up, a little painful and a little wonderful, will recognise it.
The layout adds to this. The single word "is" sits alone on its own line, as though the thawing is happening slowly, one beat at a time. The mood is simple and warm: a tiny moment of comfort, and a reminder that small physical sensations can feel like real joy.