Have you noticed that the Google Chrome browser is now marking sites without SSL i.e. HTTP sites as “not secure”. Have you checked your own site in this browser?
The change is to help users browse the web safely. Chrome indicates connection security with an icon in the address bar. Historically, HTTPS connections were labelled as secure and only required for sites processing sensitive/financial information. HTTP connections were not marked at all. Since January 2017 Chrome has been changing to the point that now HTTP sites are flagged as non-secure irrespective of the nature of the site.
Whilst this change has been known about for a couple of years at least it does still seem that many website owners are not aware and have not taken steps to install an SSL certificate and upgrade to HTTPS.
Clearly a warning like this for a website will have a negative impact on visitor confidence which is a good enough reason on its own for addressing as soon as possible, but in addition migrating your site to HTTPS encryption may help improve your Google rankings.