Mobile First - Mobile friendly websites
Your site must be mobile friendly
If your website is not mobile friendly, in most cases, you are in trouble. Google now uses the mobile version for indexation so called mobile first. If you are not sure, you can use Google's tool below.
A ‘responsive’ website design adjusts itself automatically so that it can be navigated and read easily on any device. Google is clear about the fact that having a responsive site is considered a very significant ranking signal by its algorithms. And, with the introduction of Google’s ‘mobile first’ approach to indexing content, a responsive website is now more important than ever. So it makes sense to ensure that your website is fully responsive and will display in the best format possible for mobile, tablet or desktop users.
What is mobile first
With mobile-first indexing, Googlebot primarily crawls and indexes pages with the smartphone agent. However Google will continue to show the device URL that is the most appropriate to users in Search results.
This means for instance that text showing on the desktop version but not on the mobile one may very likely be ignored for indexation and page relevancy. If there is too much text for the mobile version you can use 'show more'.
Mobile Performance is also key
In terms of performance, you need to make sure that your mobile version loads fast and in the right format. You can use Google's PageSpeed Developer Tool