As of March 26, 2018. Google have officially announced they’ve rolled out mobile-first indexing. SEO professionals have known this since back in 2017, but google didn’t officially state when they would roll this out until now.
For desktop only websites, this won’t affect your ranks: “Mobile-first indexing is about how we gather content, not about how content is ranked.
Content gathered by mobile-first indexing has no ranking advantage over mobile content that’s not yet gathered this way or desktop content. Moreover, if you only have desktop content, you will continue to be represented in our index.”
This means that your desktop site will still be indexed as normal, but a mobile-friendly version will be shown first if the user is searching on mobiles. Google are encouraging webmasters to make your content mobile-friendly.
If you have been selected for mobile-first indexing, you should receive a message in google search console, however this is being rolled out gradually so don’t fret if you haven’t!