Each OFP mission has it's own definition of what constitutes a distinguished, average or poor score for that particular mission. Poor scoring will result in 1 to 3 red X's being displayed alongside your score. Average scoring will result in 1 to 5 pearl-like studs being displayed alongside your score. Do very well and you should see 1 to 5 gold stars appearing alongside your score.
Scores can also be negative. This can occur for a variety of reasons, such as killing friendlies or not achieving objectives.
By the way, earning a high score while playing in cadet mode will result in earning less studs and gold stars than you would otherwise have earned achieving the same score in veteran mode.