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I'm assuming this is SCSS (or similar) and not CSS, which you should really note.

Here's what you want to read: http://sass-lang.com/guide

It's defining a variable, yes, to be referred to by value later on in the document.

Some notes here:

• The name is not inherent to SCSS/CSS. It could have been named $asdfg or $dogpoop or anything similar, and everything would function the same. It's just a name.

• 'Constant variable' is a slight contradiction, here. I'd google "what is the difference between a constant and a variable" to learn more.