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

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"?