Mobile First affects anyone who has a website. Mobile First has two meanings.
Build up or build down
The first "Mobile First" has to do with how you build your website.There are two ways to plan your website. You can imagine it first as a desktop application and take advantage of everything the desktop has to offer. For example, you can plan for the big screen with lots of room to let your creative energy flourish. After that, you scale it down to fit a mobile device. You might even be "watering it down". The second approach is Mobile First. In this case, you imagine your website first on a mobile device considering limiting the size of photos and giving first consideration to how fast it will load with mobile speeds. After that, you can expand to the desktop version.Read More.
Google Mobile First Indexing
The second "Mobile First" is Mobile First Indexing. This is Google's new strategy to rank your site. It means that Google will rank your site on the mobile version, not the desktop version. If your mobile version is missing important content, Google might miss that content entirely and Google search engine users won't find your site, even if they are using a desktop. I have sometimes heard website owners say that their users are all on desktops and so their mobile version is unimportant. Mobile First Indexing makes us give this idea a second look.
You can choose Mobile First or Desktop First to build your site, but the important thing is for everyone, whatever device they use, to have full access to your content. A "responsive" approach ensures that your site responds to the device, not in a "watered down" version, but providing full content.