Sentiment reflects the opinion of an author about a given topic.
Negative sentiment is, for example, a spiteful statement made against your brand, vs. positive sentiment, which would be an expression of praise, or satisfaction with your brand. Neutral sentiment, offer no strong negative or positive indicators.
Most of the mentions we see are neutral, meaning either the keyword or phrase you specified is mentioned in passing, without an opinion expressed, or that there is not enough evidence of sentiment for us to make a good judgment of positive or negative opinion.