The CSS selector finds the HTML element and applies the specified CSS properties to it. For example, to make the font size of a paragraph element you need to select the paragraph tag and add CSS property to it.
SASS is Syntactically Awesome Stylesheet, if you are working with CSS then you should learn SASS, sass is an extension of CSS and Sass is a CSS preprocessor
Mixin is used in Sass, which is a CSS preprocessor. Mixin works as a function in Sass and accepts arguments. Means you can pass arguments in the mixin.