Compass analyzes your message content to give each message a sentiment score before discarding the message content (no message content is stored on our servers). These scores can be either positive or negative and are calculated using the sentiment NPM module.
You'll see sentiment show up in Compass in a number of different places.
Coloring by sentiment
Use the settings menu in the upper right corner to color by sentiment for either the people or channels view.
There are two leaderboards based on sentiment: carebears and stinkers. Carebears is a ranked list of those with the highest total sentiment scores while stinkers are just those people with negative total sentiment scores. Total sentiment is calculated by summing the individual scores for each message over the given time period.
You can also view the sentiment scores for each person and channel using the stats view.