Mobile-first design means creating websites and apps that work great on phones...

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

Mobile-first design means creating websites and apps that work great on phones first. It loads fast, fits your screen perfectly, and makes buttons easy to tap. Navigation is simple and clear so you can find what you need without confusion