What does the stats -20 mean for a player? Is it a valuable stat?
I don't know specifically what you're looking at, but I'm going to guess it's +/-, which is a simple point differential and basically works just like the hockey stat. A player gets a + for every point scored with him on the floor and a - for every opposition point scored while on the floor. So -20 would mean that while the player was on the court, the opposition scored 20 more points than his team did.
It's basically just like hockey +/- in terms of value too: useful to a degree, but largely depends on context and nuance that the number itself lacks. A guy who puts up 20-30 points but is a minus player would suggest that the guy is potentially a defensive liability. But it doesn't account for things like the strength of the opposition or quality of teammates to give grounding to the number. For example, from the Charlotte game, Marc Gasol was +26, the 3rd highest mark on the team for that game (as you'd expect in a blowout, there were a lot of positive players. Only Oshae Brissett was a minus player, but it was just -2 and he only played 5 minutes of garbage time). But there are factors to consider in Gasol's #:
1) He played 29 minutes. This was more court time than everyone on the roster besides Siakam and Fred VanVleet. So he had more time to rack up those +s
2) He had just 5 points, so he didn't personally tilt his scales that much. But he also had 9 assists, which suggests that he did get some boost from making plays for teammates. There's also the potential that his positive value was inflated by playing solid defensively and negating opposition points. The fact is we just can't tell from the box score. Just like we can't tell, for example, how much of the minus side of the ledger might no be his fault. If he's under the basket on offence and a guard commits a turnover, there's no way he's getting back to provide any defence against the opposing fast break. But he'll get a minus just the same if they score even if he never had a hope of altering any aspect of that play.
3) He likely played much of his time with the starters (ie better players) and that would contribute to his positive momentum in the +/- column. Rondae Hollis-Jefferson was +11 in 20 minutes, but his time on the court would've been with a lot of the bench guys, which means that he would've probably not has the same opportunity to get positive differential as someone like Gasol would, even though he scored 15 points to Gasol's 5. This also ties back to point #2 in that we can't tell where his impact came from either.
I feel like it's best use is probably as something to point you in the right direction in terms of asking questions or looking at other stats and results. It's not something I would hang my hat on to say "so-and-so was +X or -Y tonight, so they had a great/garbage game". It's all about context and noise.