20/20 correction (full correction rounded to weaker 0.25 D step) is a correction you normally get from a licensed optician, which corrects your eyesight up to a degree of visual acuity that an emmetropic (or "normal-seeing") person achieves on a Snellen chart, putting your blur horizon within 0.25 D of the chart.
If you can see 20/15 or even better, this might mean that you are overcorrected (full correction rounded towards stronger 0.25 D step). (It is not necessarily considered super-human acuity, but it is unnecessary to correct to this level. Also, 20/15 isn't always a good measurement, as some people can sometimes see 20/20 with blur horizon 0.5 D away or even 20/10 with overcorrection on a bright Snellen)
It is very difficult or impossible to perform Active Focus at this level of correction, as there is not enough of a blur horizon to produce stimulus - see Distance vision, unless there is some sort of cylinder reduction or transient astigmatism involved.
A good normalized might be a further 0.25 D reduction from 20/20 correction, resulting in a 0.25 to 0.5 D undercorrection, or even a 0.5 D reduction, resulting in 0.5 D to 0.75 D of undercorrection.