Artists usually use the word value to describe how light or dark a color looks. In color science, a similar idea is called lightness.
They are not exactly the same thing. Value is an artistic concept we judge by eye, while lightness is an attempt to describe that perception with numbers.
In this tool, I use OKLab Lightness (L) as an approximation of artistic value—in other words, as a way to estimate how light or dark a color appears to us.
Further reading:
CIE — Lightness
Munsell — Value
Björn Ottosson — OKLab