Mobile-first design means building a website or app starting with your phone in...

https://wiki-coast.win/index.php/Why_Do_Mobile-First_Designs_Use_Familiar_Patterns_Instead_of_Fancy_Layouts%3F

Mobile-first design means building a website or app starting with your phone in mind. It’s all about speed—loading quickly even on slower connections. Buttons and links are big enough for your fingers, so you don’t miss a tap