The page is a tool I made to explore how to get a series of colors across all hues that have the same "brightness" and the same "colorfulness". You can read more about the background, if you're interested.
The "brightness" of colors is measured by the luminance, a linear combination of the RGB components that accounts for the eye's differing sensitivity to color. You may adjust those components as you like, or you can switch them to the sRGB values. Note that the three coefficients should sum to one for the rest of the math to work out.
With the luminance set, the gamut displays all the available colors, with hue running horizontally and chroma vertically. Blank regions are outside of the RGB gamut.
The chroma measures the "colorfulness" of colors. The white line on the gamut shows the current chroma, which may be changed by clicking and dragging. The swatches are equally spaced in hue across that line.
To avoid out-of-gamut regions, the rescale chroma option sets the chroma relative the the maximum chroma for the given hue and luminance.
The gamma factor adjusts the linear intensities for display on nonlinear monitors, like yours.