What is an “alpha value of black”?

The full context is as follows:

Use alpha values for grey text, icons, and dividers

To convey a hierarchy of information, you can use different shades for
text. The standard alpha value for text on a white background is 87%
(#000000). Secondary text, which is lower in the visual hierarchy,
should have an alpha value of 54% (#000000). Text hints for users,
like those in text fields and labels, have an even lower visual
prominence and should have an alpha value of 26% (#000000).

Other elements, such as icons and dividers, also benefit from having
an alpha value of black instead of a solid color, to make sure that
they work on backgrounds of any color.

Additionally, there are two example images.

The problem is: they appear to be using a novel definition of “alpha value.”

Additionally, their quoted hex-rgb values are not correct, and the example percentages are for their illustrated example of white text on a black background (the text says the values are for black text on white).

From what I can tell, they are suggesting that you use a transparency value in addition to your color values when specifying text and rules.

For icons, perhaps they are suggesting you silhouette the item and provide a non-white alpha channel as opposed to a fixed color matte so that you can programatically change the colors without providing new art for every conceivable value.

TLDNR; the section is poorly written and needs copyediting