In the *.mss file, how to interpret the words after the two colons?

I am currently exploring the parsing of mss files. First of all, I know that the mss file format is similar to the css format, but not exactly the css format.
In addition, I know that there are a lot of open source codes on github.
But I want to completely rewrite a new interpreter myself, one char and one char.
I encountered a problem, if a word starting with two colons appears in the selector, how should it affect the map content; for example: “::low-zoom”.
I’m sorry to say that I didn’t read the explanation of mss syntax in the wiki. After I encountered this problem, I started to check the wiki and other open source codes, so even if you explain it to me, I still have to rewrite the code.
Sorry to communicate with you using a translation tool. I am from China and am developing using C# and .NET.
Currently being tested for parsing is openStreetMap-Carto.

The syntax is cartoCSS. Have a look at Styling Concepts — carto documentation