They're calling it 'mobilegeddon'.
Who are 'they'?
In this case, various bloggers, web designers and writers who want us to be aware that from April 21 onwards, websites that are not mobile-friendly are going to be penalised in Google's search results.
As with all the changes it makes to the way websites are ranked when people search for relevant keywords and phrases, Google's not given us firm details of what the impact will be; we can only wait and see.
The most dramatic of the predictions have claimed that websites that are not mobile-friendly will 'disappear' from all search engine results.
A more likely scenario is that if a website is already performing well in searches on PCs and laptops, its position in results when people search from those devices will be largely unaffected. However, when a potential customer is searching from a mobile device, it will be the mobile-friendly websites that will appear first. Websites that are not mobile-friendly may not be shown at all in the results of searches carried out on smartphones and tablets.
With many websites getting at least half of their traffic from searches on mobile devices - for many it's much more - this could have a major impact on whether they are found by potential customers or not.
Look at your own website on a smartphone and if you have to pinch and stretch the content on the screen with your fingers to be able to read it then your website is not considered to be mobile-friendly. But if you want to be sure how Google sees your website you can pop the URL of your website into this page: https://www.google.com/webmasters/tools/mobile-friendly/
If your website doesn't get positive feedback from the test page, give your web developer a call.