The other type of rating is CR, Current Rating. This is the rating that your character earns by winning and losing matches, the one that's shown in green and red letters in the main part of the scoreboard. It's also the rating that gets you titles and mounts. Every season CR is reset to 0, while MMR remains what it was last season. So, if you ended season 14 on an MMR of 1800 and a CR of 1880, you'd have begun season 15 with an MMR of 1800 and a CR of 0.
What happens next is that as you win games, your CR increases towards your MMR. You should expect it to stabilize as you reach a CR that's near your MMR. As you win and lose games, your MMR will also shift. Your MMR aims to give you a 50/50 win-loss rate, as when that's achieved you're being pretty evenly matched.
Towards the end of season 14, Blizzard introduced something called "rating inflation" which caused CR to increase beyond MMR. This was to reward players who continued to play, rather than just holding onto their existing CR. It seems likely that this system caused the current issue -- which Daxxarri explains. The fix Blizzard put in place meant that players who, through no fault of their own, had very elevated CRs and were gaining no CR from winning matches, but losing CR from losing. The impact of this "bug fix" will be that CR comes down to where it ought to be -- somewhere near MMR.
The problem is that it's very hard to know your MMR. Player MMR is hidden, the only time you see MMR is as an average in the bottom right-hand side of the scoreboard. That's an average of your team's MMRs, not your personal MMR. While it's reasonable to assume that if you always play with the same people, your MMRs are pretty equal, if you shift around that isn't the case. So you don't know when you CR and MMR will even out.
What's more, given that beyond Conquest Points rating is the whole reward for arena, it is incredibly disheartening to gain nothing at all for winning. There have also been reports that CRs are dipping below MMRs, and players are still getting 0 CR for wins.
Community Manager Lore posted an update:
While this is an improvement, it doesn't fully repair the issue. As mentioned, the problem is that players don't know their MMR, and gaining no CR for wins feels pretty terrible. But the other option is a rating reset, which Blizzard is historically reticent about doing, especially when only a small percentage of the playerbase has an issue. Hopefully it will work itself out soon.