Plus-minus data is at the bedrock of a lot of thoughtful NBA analysis. If I tell you that the Celtics have outscored their opponents by 153 points in the 350 minutes that Jaylen Brown, Marcus Smart, and Al Horford have shared the court this season (per basketball reference), then you might reasonably conclude that these players are doing something right when they are playing together. At the end of the day, a valuable lineup needs to have a good raw plus-minus score (points scores minus opponents points scored). If a lineup of players is, over time, consistently outscored by the opposing lineups then this lineup cannot be considered effective.
The key question is what exactly does over time mean? One of the problems with looking at raw-plus minus scores for lineups are that they are heavily effected by small sample sizes. Especially early in the season, a good game or two can make an entirely average lineup look like world beaters.