Mobile-first design means building apps or websites starting with phones, so...

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

Mobile-first design means building apps or websites starting with phones, so everything loads fast and works smoothly on small screens. Buttons and menus are big and easy to tap, making life simpler when you’re on the go