Mobile-first design means building websites and apps with phones in mind first....

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

Mobile-first design means building websites and apps with phones in mind first. It makes everything load faster, so you waste less time waiting. Buttons and menus are easy to tap with your finger, and navigation is clear and simple