This particular bug doesn't itself allow someone to access otherwise protected information, but it makes some related attacks (such as Rowhammer) much easier. In addition, it looks like mitigations may be incredibly hard. Intel's press release says they hope that software can fix it (I can't help but note there's no mention of them doing anything), but that remains to be seen.
Further research is needed to see if AMD or ARM is also affected. One model of each was tested and found not to be vulnerable, but the AMD tested wasn't a Ryzen so it's still unknown if they're affected or not.