The idea of Responsive Web Design, conceptualized by Ethan Marcotte, involves the use of flexible grids, images and video combined with media queries to create web layouts that will scale to look good on any size of display. When the browser window is resized, the content adjusts to fit.
I like using Bootstrap and, more specifically, I use the responsive and adaptive starter theme for developers, Understrap (Underscores and Bootstrap) in WordPress.