Yes, specifically it refers to those graphs being mostly in a tight range. So it would might be better for most of your graphs over a month to be 150-200 pretty tightly than them being spread over 80-200 pretty evenly.

I think the word "grouping" used like that comes from shooting. In shooting, grouping refers to your bullets hitting the same part of the target, so for example if you had a very tight grouping that was centered to the right of the bullseye, the sight on the gun could be misaligned, or you might have a bad habit of pulling to the right. Whereas if you have a very wide grouping, you might just have a shitty gun or a shitty aim.