Mobile first means designing with mobile devices in mind.
This means keeping everything to a need only basis and no bloating.
In this way, we design our site to be viewed with smaller screen size and slower speed,
forcing us to be more efficient.
Many however mistake this to mean mobile only.
That is not the case.
Mobile first can easily be combined with responsive web design and progress enhancement to create a site that works great on whatever platform it is.
In this way we can give extra features and consideration depending on the type of device,
keeping only the basic features for mobile devices.
This forces the design to be less bloated and focuses on making your sites more efficient.
An example would be including richer animations, javascripts and additional features to enhance the experience on desktop devices.
Whereas the site is stripped of such features for mobile devices due to mobility issues and bandwidth constraints.
Remember, Mobile first doesn’t mean mobile only.
It means a better site.