It's basically an overall estimate of the value a player brings to their team when they're on the court that tries to remove the gaudy numbers aspect, like an advanced +/-. For example, a player might not score, but his teammates might score better when he's on the court, so his offensive BPR increases, causing his BPR to increase. Or a player might have a ton of blocks, but that's because he plays bad defense that allows opponents to get up a lot of shots, so his defensive BPR is lower than you'd expect.