Creating a great user interface design system is insanely hard and most of us don't have the skills, experience, or simply time to go about creating a decent one. Luckily for us, Google's world-class designers have created Material Design (https://material.io).
Material Design is an open design specification created by Google and publicly announced in 2014. Nowadays, most Google products (including Android mobile applications) are based on it.
Material Design was built with mobile-first and adaptive user interfaces in mind. It provides a great starting point for anyone looking to create modern user experiences that work seamlessly on desktop and mobile.
If you take a look at the official website, you'll see that it covers many topics, such as how to handle layout, navigation, colors, typography, iconography, shapes...