What is a @mixin and how do I use one in my code?

[nelsonic]

At present, the first thing anyone sees on https://tachyons-bootstrap.dwyl.com is this:

What is a @mixin and how do I use one in my project? (examples please ...)

As a beginner who has never seen Tachyons or any project that uses it, trying to decide if I should,
I need some assurance that I am investing my time wisely in learning Tachyons
So that I don't just "fall at the first hurdle" and use some other UI framework instead!

As a dwyler who is new to using Tachyons in a dwyl project,
I want to understand how the system works without any "Whaaat TF?" moments ...
so that I can be productive as soon as possible.

What Assumptions is this Making?

What assumptions does the @mixin make about the reader and that person's project?
can we please make it 100% obvious/clear so the person is not immediately "put off"?