Mobile-first design means building apps or websites primarily for smartphones,...

https://wiki-neon.win/index.php/Why_Mobile-First_Sites_Feel_Easier_Even_When_They_Have_the_Same_Features

Mobile-first design means building apps or websites primarily for smartphones, so they load fast and work smoothly on smaller screens. It uses big buttons and easy-to-tap links, making everything simple to navigate with your fingers