PostCSS and SCSS both are CSS Pre- Processors (means you write CSS code using some tool (PostCSS/SCSS) specific style and it will get converted to CSS after a build process ) .
Using CSS Pre-Processors give you a lot more functionality than writing CSS in traditional style (Like auto-prefixing of vendor specific classes, complex calculation inside classes, using Global/local variables, mixins, functions, nested classes .. a lot more ).
The main difference between SCSS and PostCSS is the light nature of PostCSS. Means SCSS installation always carries all the functionalities it can provide while PostCSS is more modular in approach and you add functionalities that you need only in form of PostCSS plugins. (Like you need Auto-Prefix add its plugin).
Here's a good overview of PostCSS with the problems it solves while compares to SCSS & SASS. ashleynolan.co.uk/blog/postcss-a-review