if +, possibilities are,
1) You scored
2) Your teamates scored
3) The other team had hard time to score because of your defense.
if -, possibilities are,
1) You can't score
2) Your teamates can't score either
3) The other team scored
In Lin's case, he didn't score that high but his number is a big +, it means when he was on the court, his teamates scored, and/or the other team had a hard time to score because of defense.
In Harden's case, he scored high but his number is -, it means while he scored, his teamates did not score a lot, and/or the other team scored a lot because of poor defense. Based on my eye ball, his teamate actually scored quite a lot, so the conclusion is more likely to be that the other team scored more when he was on the court because lack of defense. He is usually NOT on a man (I'm not sure if he is supposed to have his man).