Mobile-first design means building apps and sites mainly for phones and...

https://wiki-stock.win/index.php/What_Does_%E2%80%9CSecurity_Must_Remain_Invisible%E2%80%9D_Mean_in_UX_Terms%3F

Mobile-first design means building apps and sites mainly for phones and tablets. It focuses on speed, so pages load fast even on slower connections. Buttons and menus are big and easy to tap with your finger, making everything feel natural