A CSS framework, also known as a web design framework is a pre-prepared library that is meant to allow for easier, more standards-compliant styling of a webpage using the Cascading Style Sheets language. Just like programming and scripting language libraries, CSS frameworks (usually packaged as external .css sheets inserted into the header) package a number of ready-made options for designing and outlaying a webpage.

Now here they are ....but wait i think you need to read this first Which CSS Grid Framework Should You Use for Web Design?