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

https://mighty-wiki.win/index.php/What_Makes_a_Mobile_Interaction_Feel_%22Natural%22_to_Users

Mobile-first design means creating apps and websites that work great on phones first. It’s all about making things load fast, so you don’t wait around. Buttons and menus are easy to tap with your fingers, and navigation is clear and simple