Building one interface that adapts its layout to any screen width, from a phone to a wide monitor, instead of maintaining separate mobile and desktop versions.
More than half of traffic is mobile for most products, and the mobile version is what search engines index. A layout that only works on a designer's monitor loses the majority of real visitors.
We design and build mobile-first, test on real devices rather than only in a browser emulator, and treat tablet and large-screen layouts as deliberate decisions rather than whatever the grid happens to do.