Sentiment analysis tries to decide whether a piece of writing is positive, negative or neutral. It is useful for scanning large volumes of feedback, but language is slippery: sarcasm, mixed feelings and context routinely fool it.
A sentence praising one thing while criticising another, or a complaint phrased politely, can be misread. Treated as a rough gauge over many messages it is helpful; trusted as a precise verdict on a single one, it often misleads. Knowing its limits is part of using it well.