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

https://atomic-wiki.win/index.php/How_Do_I_Check_If_My_Site_Is_Truly_Mobile-First_or_Just_Squeezed_Down%3F

Mobile-first design means building websites and apps with phones in mind first. It focuses on speed so pages load quickly, uses big, easy-to-tap buttons that fit your fingers, and keeps navigation simple