Net Rating is a team's Offensive Rating minus its Defensive Rating. It boils a team's entire scoring margin, adjusted for pace, down to a single number: how many more points a team scores than it allows per 100 possessions.
The league average is 0.0 by definition, since one team's offense is another team's defense across the whole league. Anything comfortably positive is a good Net Rating; the best teams in a given season typically land in the high single digits or better.
For projecting how a team will play going forward, generally yes. Win-loss record can be skewed by close-game luck (a team that's 8-2 in games decided by 3 points or fewer isn't necessarily better than its record suggests), while Net Rating reflects the actual scoring margin a team has produced, which tends to be a steadier signal over a larger sample.