Desktop first vs mobile first approach
The difference between those approaches is how you architect your CSS code, especially the which media queries are used.
Desktop first
Desktop first approach means that the code is written for the desktop
version of the website and later "fixed" so it looks good on mobile devices.
Dead give-away of such code are max-width media queries:
.post__title {
font-size: 8rem;
@media (max-width: 40rem) {
font-size: 4rem;
}
}
Mobile first
This approach requires you to first code the website for mobile and then make appropriate changes for desktop.
Most media queries will have the min-width condition:
.post__title {
font-size: 4rem;
@media (min-width: 40rem) {
font-size: 8rem;
}
}