What is desktop first and mobile first design approach?

• Desktop first : General selectors and styles designed to make the site look good on DESKTOP screens defined globally. But they affect all devices, and must be overridden by max-width media queries targeting minimum screen size
• Mobile First : General selectors and styles designed to make the site look good on small MOBILE screens go here. But they affect all devices, and must be overridden by min-width media queries targeting maximum scrren size

In desktop first approach the media queries will be written with respect to max-width whereas in mobile first approach media queries will be written with respect to min-width