Breaking this word up into smaller parts, lets start with the phase 'Style Sheets'.
Style Sheets: During the earlier days of the internet, HTML only allowed for very little style of it's web pages, and only allowed for inline styling, written within the same file. Nowadays, these style sheets, are files that segregated from HTML files, and can be easily added anytime.
Cascading: This word represents how the styles are organised, when both styles for the same selector is made, the latest change to that style will be the one most prominant - the one that actually gets used.
If you are completely new to web development, please head to W3School CSS
A Complete Guide to Flexbox - CSS tricks guide to flexbox is one of the best and easiest to understand guides out there
Flexbox - An nice animated guide to Web Designers companion for responsive designs
CSS Animations - Great playlist from Devtips (definitely recommend watching a lot of his videos)
Pseudo Class Selectors - Gain Mastery manipulating the shadow-DOM
Smoother & sharper shadows with layered box-shadows - Create smoother box-shadows with some techniques from Tobias
Relearn CSS layout - If you find yourself wrestling with Flexbox this will be a good refresher/guide