Mobile-first design means building websites and apps to work great on phones...

https://wiki-stock.win/index.php/How_to_Prioritize_Content_on_a_Small_Screen_Without_Cutting_Features

Mobile-first design means building websites and apps to work great on phones first. It focuses on fast loading, easy-to-tap buttons, and simple menus that fit small screens. This makes browsing quick and smooth, even on the go