Linda B's poem is about how much of a person stays hidden. On the inside, her heart rests at a calm sixty beats a minute, quietly gathering experience the way a tree adds rings. Those layers build up out of sight, and only a small glimpse ever reaches the surface. What does show is a mix of sweet and bitter, as if life were seasoned with a pinch of salt and then a great deal of sugar.
The ending turns the poem toward something more personal. Her heart is racing, yet nobody would guess unless she says so out loud. The speaker seems aware of the distance between what she feels and what others can see, and she suggests that speaking is the only way across it.
The mood is gentle and thoughtful, with a little tension underneath. The slow, steady opening gives way to that racing heart, and the poem quietly points out that we have to tell people how we feel.